Haven't got the spreadsheet in front of me but from memory...
I'm assuming the tips are carrying at least 125 lb of the fuel, hence the MTWA is 3725 lb. With the tips empty, I'm down to 3600 lb MTWA
2610 lb aircraft
720 lb people
70 lb baggage
20 lb anti-ice
75 lb emergency equipment (mostly the 4 man dinghy), tools, cover etc.
230 lb fuel = 38 USG (of which at least 21 USG must be in the tips)
Total 3725 lb.