Booking from SAN means you have to use the Star Alliance award chart. It would be a lot fewer miles to fly a pure Singapore routing from LAX as you can follow for the Singapore Saver award chart (one-way in F is 105k vs 135k). Also, if you’ve only got 90k and want to use them up you may be better off flying one way on Singapore and the return with a different currency. You can still do a stopover on a one-way ticket for $100. So you could go from LAX-ICN(stop)-SIN for 105k SQ and return via Korean which is 95k from Southeast Asia or 80k from Korea to the US in F since you have Chase UR.