Hi Andy,

You wrote:

"A lookup table is probably easiest."

You are 100% right. It's much more than easiest. It's the best solution by far. If you decrease the internal input range (using 80C535's DAPR register) in order to increase the resolution, the linear conversion (whatever code is used for calculation) from HEX to packed BCD or direct 7 segments is useless.
It worth to waste some bytes for lookup table.

Regards,
Silviu