Hello C,
Welcome to the forum.
You say the outputs are not grounded, is the stimulator grounded? If not, then you are struggling without some form of isolation. If the stimulator is grounded then you can measure the o/p voltage with reference to its ground, and from your description you need to do it on both outputs and compare the result.
Ditch R_t
Connect R_1 and R_2 (A1) to the lower output
Leave R1 and R_2 (A2) where they are
(Please, on your next diagram give your resistors unique identifiers, so R1, R2, R3, R4)
3950 and 50 ohms is too low for 300V, at that voltage they will have 7.5mA (75 times what you are shocking your mice with) and their load will swamp the load you are measuring. They will also dissipate 22.5W, which is rather a lot. I suggest a total resistance of around 3M Ohms would be more reasonable, I leave you to calculate the values.