Very individual thing i assume. I don't have any means to know how fast the testosterone in the gel gets metabolized by your body. Because you use Ethanate on top of it.
I've always taken tests just couple days before new injection.
So with 1 shot every 10 days i've might taken lab test at day 7 or 8.
In reality, are you aiming for some sort of self medicated small cycle or simply after a long term trt and just messing with it a bit?
The half life of ethanate is roughly 6-7 days. Bluntly it would translate roughly like: day1 = 250mg, day7 = 125mg, day14 = 62.5mg and so on. After two weeks in theory you should still have 62.5mg left. Sounds good to me.
Edit: Fixed day14 half life amount from 65.5 to 62.5: I just can't do simple math...