Frank, I run a Saito 100 in a H9 P51 (inverted, with a similarly positioned tank). With this setup, my idle was a very reliable 1800 rpm, running either 15/18 Omega or 20/20 Coolpower fuel and an OS Type F plug.

Once your engine is broken in to your satisfaction, I would recommend checking the tappet clearance, trying a fresh glowplug and fresh fuel (your 15/18 mix should be fine) to establish a baseline. Then, try leaning off the low speed needle as much as you can while still retaining the ability to transition at a normal rate to full throttle from idle. I run mine absolutely as lean on the idle as I can get it before transition suffers.

An additional hour or two's running generally will improve the engine's behaviour somewhat.

Good luck.