With .38*50, I'd just round that up to .4 and just make up 20 over the next streets.
Are you playing draws with the intention of making up a certain amount on later streets? I don't mean to pick, this just sounds a bit odd to me. When you make the call you're expecting to make the money on later streets based on your strategy for later streets. Like, i've done it myself, thought "OK i just need to make a few extra $ and this set mine worked out ok."

Basically, if i'm calling in position, i expect him to bet or check/call down. Out of position, things are going to be a little tougher unless my hand is pretty well disguised.

I'd be rounding the 0.38 to 0.4 yeah, but in truth, i'm rarely gonna know my equity to 2 decimal places.

Maybe I just tried to over-simplify it for myself to make things quicker, but perhaps over-complicated it in the end.
Just whatever makes you feel most comfortable. I don't like learning things to memory and I dont like using formulae (more to remember) so it doesn't work for me. What I would say is not to get too caught up on place accuracy. Your 'minimums' are a good idea in that they are nice easy numbers to work with, but so I suppose are the %s we deduce in a 2% per out per street approximation. I'd just play around with things, see what you are best at.