adding maltodextrin to sugar/salt mix

How do you calculate how much maltodextrin to add to your carb mix?

During training i'm totally fine with high carb homemade mixes of just sugar and sodium citrate. However when in a race and just going as hard as I can for 4-5h I find I can't get the 90g/h of carb mix down. It kind of makes me want to gag and all I can get down is water. From what I read the solution is to start adding maltodextrin to take away the sugary taste, wondering how much I should add?