It’s a bug they should know about, it was mentioned for example here: TSS estimated on calender incorrect
I would probably just estimate it by finding out what tss it gives you for about 10 hours, and then multiplying by however many hours you think you’ll do.
Tbh I’m not sure if the exact figure matters, it will basically be ‘a lot more than other rides’. I think the whole tss thing works best for typical road races and not so well for other events.