I have spent the day studying this business of reading bits from first principles and I think I am learning a little.
It seems to me the timing from bit to bit only needs to survive for 8 bits because the start position gets zeroed with every new byte - that's the purpose of the start bit.
And if that's correct it seems to follow that it would be a good strategy to arrange the first "delay" so that the read of the first bit value will happen soon after the start of that bit interval. Then if there is a positive error in the number for the "delay" between bits each successive read will take place a little further along the actual bit interval. But so long as the interval isn't too big it won't fall off the edge of the bit until after the 8 bits have been read. And it can't fall short.
The advantage of doing it this way (it seems to me) is that there is more control of the inevitable error compared to trying to find the centre of the bit interval and work out exact timings from bit to bit. For example it is easy to calculate the theoretical delay but in practice the actual time will always be longer than the number you choose. And it would be illogical to aim for a total delay that is shorter than the theoretical value.
If this is all BS please tell me.
...R