Adam, I think it's as simply as equal jamming sugar to strawbs, pound for pund. Crush strambs with sugar and gently heat till sugar has dissolved. brign to the boil for 4 mins (keep stirring) remove from the heat and drop a bit on a cold plate, wait to cool and see it if wrinkles when you touch it. If it does it's ready to pot, if it doesn't wrinkle, return to the heat for a minute or too long and repeat the 'wrinkle test' until it wrinkles.

Hope I didn't lose you there

Has to be jamming sugar not normal as strawbs don't have enough of their own pectin

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