I think the reason you're not supposed to stop urine midflow too much is that it can cause an infection?
I go to the loo before my shower, and then do my exercises in the shower, push out like I'm trying to pee, and then pull those muscles 'upwards' like an elevator (well, the elevator imagery works for me )
The pushing out helps me make sure I'm using the right muscles and if some pee comes out, well, I'm in the shower so it's all good

Hope that helps, all the best for finding something that works for you!