Presumably I will have to create a specific variable to track the number of variables created per event along the lines of myvar = runner_number at each t90, t60, t30, etc. time event?
I can't see why you would need to track the number of such variables, but then I don't know the details of your strategy. If you need help in analysing it deeper, please make a trigger request.
Incidentally, would the following work given the previous scenario of the t90 favourite being different to the t60 favourite?
No, it wouldn't work, specifically this construction:
t60_market_id_back_price / t90_market_id_back_price
will try to get the back price of the same selection (a selection with the same ID) as remembered in the t90 and t60 sets. If this selection was not remembered in either of these sets, the expression will not be parsed correctly. So this is not equal to the ratio of the price of t60 favourite to the price of t90 favourite.
Instead you could just remember the price of the favourite at 90 and 60 minutes before the off and then divide one by another, e.g.:
Trigger 1:
set variable t90fav to r_1_back_price
Condition: Market's minutes before the off is less than 90
Trigger 2:
set variable t60fav to r_1_back_price
Condition: Market's minutes before the off is less than 60
Trigger 3:
t60fav/t90fav etc.