I've never done a micro grow but the rule is that for a normal plant, one gallon of soil per foot of plant height is adequate. For a plant that is being scrogged or otherwise trained, give it a bit more- just use your judgement- If you have a single stem plant that is a foot tall there's a gallon, but if it's trained to 4 main stems, give it like 3 gallons or so.
Soil start to finish- when I was doing soil I used that compost mix I gave you start to finsih, but it WAS very very 'hot' for seedlings and I flushed the living CRAP out of it before planting.
I'm now a happy coco fan, and I start my seeds in rapidrooters (composted tree bark) and then go to coco. It's inert - no nutes included, so you have TOTAL control.
For all the shit talked about miracle gro, that is to me the biggest issue:
CONTROL.
I want to be able to give my plants exactly what they need and not have to sit there scratching my head about what is in the soil, or of what HAD been in the soil, what is left by now (with MG this is likely to be unusable salts precipitated out from interactions among ferts). With the coco, it is super easy to control and impossible to overwater, so you can flush as often as you like. In fact, overwatering by 10% each time is actually RECOMMENDED for coco!!! Sez my fertilizer guru.

Okay tangent. sorry... a lot of people swear by ProMix BX which you can find virtually anywhere. I'm not personally a huge fan because of my [almost illogical] hatred of All Things Peat Based... haha but I've seen many a beautiful grow using that stuff, and you can do it too I'm sure.