In short, the formula looks like this:
Dose in MG / concentration in mg per ml = dose in ml
But let's make the math easier. How about you do 300mg test with the 150mg tren? Yeah, that's a great plan!
100mg of 250mg per ml would be 40 units aka .4 ml.
Easy math there:
100 / 250 = 2 / 5 = .40
Don't believe that? Bust out the calculator!
That tren dose is super easy though.
You want to do 50mg at a time, so:
50 / 200 = 1/4 = .25 ml per shot.
And to address your original dose test dose:
83 / 250 = roughly 1 / 3 = .33 ml aka 33 units.