The Guidi 992 is the derby form of the 994 or 995, right? I find that those size 1 size small, in general, but that they feel really comfortable after a couple of wearings, in any leather. For reference, I wear a size 45 in all the 995 and 994 I have or have tried on, and wear a size 44 in Margiela GATs and 43 in Common Projects.
Yep, the 992 is a standard lace up derby. To complicate things I have a slightly wide forefoot, which I would hope works well with the very rounded toe box on the Guidi's. 44 in CPs were too narrow in the front for me so sounds like I'd need at least a 44.5 or 46 in the 992. Do you know what size buttero (sneakers) you need? I wear a 44 which is slightly too long but good on width.