SEiW westi busa using std bike clocks. Std westfield sender unit. Does anyone know the correct resistor to use from the sender unit to dash guage for a close calibration value?

Dunno.

But I used a busa sender and just bent the arm so that it reached full travel at both full and empty ends - worked a treat!

The Busa sender resistance is 130 ohms empty and 12 ohms full. A Stewart Warner 385AB sender is 250 ohms empty and 30 ohms full. With a 250 ohm resistor placed in parallel across the terminals of the sender the bike gauges will show empty when the tank actually is empty and about 90% full when the tank is actually full. 'Not perfect but pretty close. I was more interested in knowing if I was going to run out of gas than confirming that the tank was topped up. The SW385AB lever type sender is about the cheapest you can find ,also.