I think you are overfitting the curves too closely from 99 to 03 and using the URA price index and STI at different scales (e.g., STI from 1000 to 4000, but URA PI only from 100 to 180). While we can see that both moves in the same direction, it is very hard to extrapolate on the two variables, percentage of decline and the timing of it.

If we say that STI is around 1000 in mid 98 and it went up to 3800ish (?) at its peak, it is an increase of 280% and assuming 180 is the peak for the URA Price Index, it is an increase of 80%. Therefore, using basic proportions, a 25%ish drop in shares price will now only mean less than 8% drop in property prices.