"Is it necessarily a problem for the flush if I have bowel movements on the evening of the flush itself, rather than only on the next morning?" No.
"If I reduce the Epsom Salt dosage to 3/4th of a tablespoon each time, does that reduce the efficacy of the flush?"
You should find the dose hat feels good for you !
The original recipe is just a start ... once you have it, you can tweak the recipe the best it feels for you.
Listen to your body.
The main advantage of a large dose of Epsom Salt is that it flushes all potentially toxic bile and sludge and stones from your intestines, so you avoid re-absorbing it back into the system.
Normally, when not flushing, our intestines re-absorb up to 30% of all excreted bile back into the liver.
During the liver flush, we don't want anything being re-absorbed, so we want it very fast out.
Fast out is made possible by epsom salt.
You should take the smallest possible amount of Epsom Salt, which is still sufficient to flush all bile and stones from your intestines.
Smallest possible amount of Epsom Salt for me and for you may be a different amount. I may need double as much as you.
This amount may change with time ... as your intestines become more sensitive to Epsom Salts .