Best time to do an H1B transfer


I’m currently in the process of getting my first H1B from scratch with Company A (already got I-297 and waiting for approve (I-797)). I’m abroad and have no any authorization to work in the USA at the moment.

The thing is that I’m not really want to work with Company A and would like to do a transfer ASAP. Of course, I plan to reimburse all expenses to Company A (just ethical thing).

My question is what is the best time to do a transfer? As I understand, the safest way is to work for Company A ~2 months, get payslips and then do a transfer. But is it possible to do it sooner?
In any case, I will able to start with Company A not from the 1st of October but a bit later (it will be only November/December due to personal reasons). Will it be possible to do a transfer in a period from 1st of October to 1st of November before I will start working in Company A? Will that raise my chances for RFE significantly?

Thanks in advance for your help!