you want the pre funding valuation: post funding is valuation plus investment normally so: $2m and your share of that - 2.25% if I've done my sums right. You could probably ask for a bit more for being willing to take a punt on them raising the 500k funding and hence your risk being higher. You can also dispute the $2m valuation (but obviously if you push it down too much then the 500k round will like your valuation better and you will be diluted more anyway) - given most valuations at this stage are "how long is a piece of string".

If your round and the 500k are very close together the investors for the 500k may expect your terms to be the same as theirs.

also with the best will in the world their plan is just a plan and it won't pan out like that (speaking from very similar experience). Moving to SF is hard and expensive unless they already have visas or are US citizens, raising 500k is hit and miss (especially in the UK) and their hiring plans will take longer than they want.

Also from personal experience (on the other side of the coin ) I would suggest that you will mitigate your risk much better by finding someone else to come in with you now for say 60k which will keep them going much longer than the 4 months you will be buying them - because unless the 500k round is well advanced already 4 months is just long enough to make them desperate when negotiation time comes for the bigger round.