@Mr. Sauga and I just had a conversation about this yesterday. He likes to run his silica in PPM, and starts at 10ppm with the second set of leaves, then bump it to 30, and then to 50ppm for the rest of the time.
What I discovered using Hydrobuddy's calculator (which is what MrS uses) is that my MSA is a pretty low concentration (probably why it was less expensive than others), so I may be underfeeding with it. I'll see what the actual weight is per milliliter (since Hydrobuddy works in weight rather than volume) and get a more accurate dosing schedule.
And it's hard to overdose your plants with Si so you should be okay starting on the high side as long as you're not using a 0-0-4 version. That will throw out your NPK ratio.