8051 TH0/TLO flag values
Loading 0x05 into the high-byte of the timer is the same as loading 0x0500 into the entire timer, and that would by a count of 1280 (decimal). So if dividing your clock by 1280 gives you 250 microseconds, then your clock is giving you 0.1953125 microseconds per machine cycle (pretty fast for an 8051). I suspect that your machine cycle might actually be 200 nanoseconds, which would make your reload-time closer to 256 microseconds.

A timer value of 0x0080 would be 128 decimal. Multiply that by 200 nanoseconds and you get a a delay of 25.6 microseconds.

If you haven't yet noticed, 0x0080 is exactly one tenth of 0x0500.