This seems specific to 3.5 so far. I may have previously used this in previous Moodle versions without problems. In defining unit multipliers, I have defined J (joules) with a multiplier of 1.602e-19 on the primary unit of eV. However, when the question is saved, the multiplication factor changes (without any intervention from me) to 2.0e-19. I have tried changing tolerance type from decimals to significant figures, and back but it seems unaffected. This is messing up auto-grading as students are incorrectly being marked incorrect.
This is exactly the same is this issue: MDL-69344 (numerical and calculated question types share the relevant bit of code). The problem is explained there. There is no easy solution.