The problem with maltodextrin is that it has a DE (dextrose equivalent) rating which relates a unit of measure of it to a unit of measure of glucose (dextrose - same sugar, 2 different names). The malto number always indicates that its less good of a reducing agent than glucose is (but a much better stabilizer if you're making over 20PPM and choosing not to gel cap it). The problem lies in the fact that any malto we can buy is devoid of a DE equivalency as its "mixed bag" given its main uses don't need it. Without a DE number you have no clue how much you actually need to reduce properly.
Kephra provided a formula that will work for any mixed bag malto you get your hands on. That formula is as follows:
PPM * Volume * 0.014 where volume is in liters
So as an example, you have 1 liter and have 40PPM to reduce so...
40 * 1 * 0.014 = 0.56 grams of mixed bag malto
For Karo, Kephra's formula (1:1 karo/vodka)
PPM * volume(ml) /16000
So if you have 20PPM and 1000ml, you'd need to add 1.25 drops of dilute karo (I always go a little higher for safe area like round up and add another drop - it won't hurt anything).
Not only does the karo/vodka thin it so its easy to use an eyedropper, the alcohol prevents the karo from evolving life - it'll stay good in a tightly closed dropper bottle even on the table basically forever. A little 2oz dropper bottle full will last damn near a lifetime unless you're making swimming pools full of Colloidal Silver (wink).
I do need to warn you about something though. At room temp (thats 75F, NOT 68F), you can only dissolve maybe 22PPM of silver in water. Beyond this at this temp, you will start precipitating out the silver oxide which will now stay silver oxide forever because it won't reduce because it needs to be in solution to do so which it no longer is. You may still have 21-22PPM of dissolved silver oxide. In other words, if you're making 40PPM at room temp, that would be bad. You'd get to 21-22ppm and the remaining 18-19 PPM will stay silver oxide even after reduction and worse yet, the particle size may be small enough that the silver oxide floats in water for a very long time before settling out.
At 150F, the dissolution limit for silver oxide is about 40PPM.
If you're making more than 20PPM you need to process at a higher temp and you need to add the reducing agent at the beginning of the run so you get continuous reduction so you never exceed the solubility limit for the temp you're running at. The complication is that to get a reducing agent to reduce quickly enough, you need to be running at a higher temp (say 125-150F). At cooler temps, reducing agents work but slow as a snail.
I routinely make 120-160PPM Colloidal Silver using the method addressed above. Then, after your run is done, because the last few minutes of IS you made hasn't reduced yet, you need to remove the jar from the heat, put a cover or cap on it and let it sit 10-15 minute for all the IS to reduce to Colloidal Silver BEFORE you'd choose to gel cap it if you want to.
Some suggest adding the gelatine at the beginning but I prefer to add it after the run is done simply because adding it at the beginning slows the reduction process down and after maybe an hour you'll have to stop the run, flame clean the anode and continue (it gets a bit gunked up). To each his own. Whatever works best for you. There are lots of ways to do it.
The equation to calculate run time is more correctly referred to as Faraday's law of electrolysis. For silver, 15ma minutes of current (15ma for 1 minute, 1ma for 15min,...) will pull 1milligram of silver into solution.
So to calculate run time its simply
PPM * volume (liters) * 15 / <the current you're running at>
or
PPM * volume (liters) * 0.015/<current you're running at in AMPS>
or as you also said
PPM * volume (ml) * 0.015/<current you're running in ma>
The last equation to me is confusing because you're taking a ratio of current to current which provides the multiplier and you're using amps/milliamps and then because milliamps are 0.001amps, that cancels the volume in ml to basically be liters. Why bother?
40 * 1 (liter) * 15/10 as an example is 60 minutes
40 * 1 (liter) * 0.015/0.010 as an example is 60 minutes
40 * 1000ml * 0.015/10 as an example is 60 minutes
See how it works. THIS is why you need a constant current. If you don't know what current you're running at you won't be able to compute how long to run for the PPM you're wanting..
The other thing to watch for is to pick a current thats properly chosen for the size of your anode. If its silver wire, you may well only be able to push it to 5-6ma before you start running into trouble at higher temp and if at room temp, it might be more like 2-3ma since at lower temps, things go more slowly. If you've got a 1ozt bullion bar, you might be able to do 5-6ma at "room temp". At 150F+ you should be able to push it to 15-20ma. Though it doesn't need to be said, the higher the current you can safely run with, the faster what you're making is done.
If you run at too high a current, some of the created silver oxide will precipitate rather than dissolve. You really don't want to do this (wink).
Just to reiterate though, if you're making higher than 20PPM, you NEED to add the reducing agent amount for the PPM you're making (plus a little for safe area - it won't hurt anything) at the beginning before you start the run and you need to be running more like 125-150F to make sure the reducing agent reduces fast enough that you never accumulate more than 30 or so PPM in solution before its reduced. At 150F, you've got time and at that temp, reducers reduce quickly (minutes, not seconds).