As part of an engine ignition project I need to calculate an advance value
using a lookup and a measured RPM. Seems simple enough but I've come
across a small problem.

The time for 180 degrees of crank rotation is measured giving a 16 bit
number which is used in a lookup to give an 8 bit advance number
corresponding to a range of 0-45 degrees of crank rotation. This number
then must be converted into time, based on the RPM to feed into a delay.

The numbers look friendly - dividing the 180 degree time by 4 gives the
time for 45 degrees, dividing that by 256 then gives the time for an
advance number of 1 so then just multiply by the advance number. Since the
division is by 1024 overall, I intended to just shift the bits right,
saving a slow division routine. The problem with the 'divide by 1024 and
multiply by advance value' process is that a lot of the precision is lost
if I divide by 1024 first. Multiplying by the advance number first would
require 24 bit maths routines.

Anybody got any cunning ideas? The calculation must obviously be quite
quick

Thanks,

Mark.