I don’t have first-hand experience with this chocolate, but I’ve found that the nutritional content labels can be very useful when making substitutions in recipes.
You can compare the sugar content of your chocolate to the sugar content of the chocolate in the recipe. You take the weight of bittersweet chocolate, say, called for in the recipe. Get the info off a bar of bittersweet to see how much of that weight is sugars, and how much is non-sugar chocolate components. Using the info off your 90% bar, calculate how much of it you need to get the required amount of non-sugar chocolate components. Make up the difference between that and the weight of chocolate called for in the recipe with sugar. This will be a bit approximate, because the fat percentages of the bars probably differ. Also, if your recipe doesn’t call for any granulated sugar, the issue of how to incorporate the extra sugar becomes tricky.
My guess is that the 90% is a better match for unsweetened chocolate. To use it for that, reverse the process. Use enough of the 90% to get the non-sugar chocolate components called for in the recipe. Reduce the sugar in the recipe to compensate for the extra sugar in the 90% chocolate.
I’ve had good success with this general approach to moving sugar and chocolate around in recipes.
If this approach interests you but the kitchen+calculator concept doesn’t appeal to you, I’m happy to help with calculations.