Hey rache
You could go for 60's or 70's in theory. 60's would be alright, certainly a lot softer than the 90's you are on right now. Chances are you would love them! Just because they are so easy to compress. But, you are on the 10kg margin, the springs wont last as long or be as good for you in the long run I dont think. I think because 60's would be so soft you would progress very fast and need to upgrade quick too. Particularly because you are used to the heavier springs. 60's wont be too good for running also.
I say go for 70's. You will be going down from 90's so you will still (and have) felt the difference. Its quite a lot. Because you are downgrading you should be totally fine on them and I think you will progress fast so getting 70's would be the best option for you. The best compromise between progressing really fast and the springs being right for you in the long term.
