Mitsubishi mini-split sizing

PaulDon

I've got 3 bids (all Mitsubishi diamond contractors) to install single zone Mitsubishi ductless in my living room (360 sq feet + 2 large old windows facing south + 2 windows (E and N side) + old brick fireplace(cover with board) + 170 sq' kitchen through door + opening to the hallway (will be closed with drapes + no insulation in the walls/ foor + new insulation in low attic).
2 bids were for 12,000 BTU simply based on the sq footage size, and 1 bid was for 24,000 BTU in order to cool the whole area on hot days, possibly the whole house of 1400 sq' with a fan. I need AC only during heat wave (90F+) in northern CA.
I searched what the Japanese market is recommending for sizing, and found out 18000 BTU for 350 sq' (for wooden structured, no insulation houses). But here in US, 12000 BTU for up to 500 sq' except more BTU needed when kitchen, large windows, south facing etc. involved.
2 recommended WR series with 16 SEER since we probably use AC 30 days/year max, which is fine.
I think 18000 BTU is the one, but any thoughts from your experience?

pecmsg

Manual "J" Heat Load / Loss calculation