If you use Simbrief, I've been playing with the latest version that includes a takeoff performance calculator.
It doesn't have preset data for the F.28 but it does have it for the F.70 and can give a calculated Flex Take Off temperature.
I've been loading data for my F.28 into that, then bumping up the weight by 10 per cent or so to account for the lower power/weight ratio of the F.28.
So far it's always spat out Flex temperatures in the 40-45 deg.C range.
Using the thrust index numbers to go with those temperatures certainly seems to do a good approximation of a Flex takeoff - less of the frantic 'pocket rocket' climb that you get at full rated thrust. I feel you could bump the assumed temperature even higher but without the proper data it seems a good compromise.