Kelly's criterion for slots

So sometimes I do little slot special offers (for the fun of it mostly) but I really don't know how to gauge variance in slots. Is there a way to use something similar to Kelly's criterion when playing slots?

For example tomorrow there's a deal on bet365 where if you wager 50 euro you get your winnings doubled or, if you're down after the session, you get a few free spins. This is definitely +EV and the EV definitely increases with the wager (since if you make bets of 5 euro for example and you hit big the doubled winnings will be huge) but I've no idea how to estimate the variance and decide on the wager size to optimize EV and variance.

Any ideas?

Quote: adekoon

For example tomorrow there's a deal on bet365 where if you wager 50 euro you get your winnings doubled or, if you're down after the session, you get a few free spins.
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Welcome to the forum.
Are the free spins more valuable if your average bet is higher?

No, it's always 5 free spins for 20 cents, but I think even without the free spins the deal would be +EV, since doubling your winnings would be huge for higher wagers.