How to calculate spread on gambling odds?

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I'm trying to calculate the spread on a bet.

People are telling me it's either 10% (absolute) or 17% (relative).

I'm failing to see how to get either number. Bet is below:


Context: Which stock will end up more (%-wise) by the end of 2019. The options are either to buy Stock A at 6:5 or to buy Stock B at 2:1 (against)

I see 6:5 on Stock A side payout, which means if you bet on Stock A you get 1.2 rewards, if you bet on Stock B you get 0.83 rewards

I see lay 2:1 on Stock B, so if you bet on Stock B you get 0.5 rewards, if you bet on Stock A you get 2.0 rewards