Accepted answer
At 3.33 mg/mL a 0.25 mg dose is 7.5 units on a U-100 barrel and a 1 mg dose is 30 units. Volume is dose divided by concentration and one unit is 0.01 mL, so the unit count is dose ÷ 3.33 × 100. Both land in a readable part of the barrel, which is what choosing the volume deliberately buys you.
Aim for a dose volume somewhere between about 10 and 30 units on a U-100 barrel and the reading problem disappears.
The other direction: 10 mg in 3 mL is 3.33 mg/mL, and a 0.5 mg dose becomes 0.15 mL, or 15 units. More barrel, easier reading, and a larger fraction of the vial volume lost to dead space across the same number of draws.
Worth being precise here: vial headspace is the hard constraint. A nominal 2 mL vial typically holds a little over 2 mL to the shoulder; adding 3 mL is not an option and attempting it wastes the lot.
Insulin syringe barrel graduations are typically 1 unit on a 0.3 mL barrel, 1 unit on a 0.5 mL barrel and 2 units on a 1 mL barrel, which is why the barrel size changes what is readable.
Do not change the diluent volume between vials of a titration without recalculating; it is the commonest source of a ten-fold error.
Write the concentration on the label at reconstitution, in units per dose.