Horton starting once per week from April and averaging 5 IP/GS would have him between 130-140 by end of season. He'll likely be shy of that 5 IP/GS mark in Iowa if last year is any indication, so if the target is roughly 130 IP then he's basically on track to do that without any adjustments in a minor league season.
So with that baseline, how do we adjust for any MLB impact. As a reliever, this is done by default, pitching even only in September in relief would likely leave him with the bullets needed to be part of the MLB pen through any postseason run. As a starter, the first consideration is if he can still start 1x/week, Counsell + the existing depth would leave me optimistic that would be still possible, so then you need to account for any postseason impact. He's very unlikely to be a 2x/series starter, so you maybe skip him or delay him a few days once a month or so which gets you pretty close, assuming he's a 5 and dive starter this year at the MLB level(a fair assumption in a pennant race/postseason). There are other permutations, but they're of the type that you can't really plan workload around(Horton goes supernova on Iowa, injury ravaged MLB rotation).
So to tl;dr it, if by the end of April Horton looks like a guy who is gonna force his way to the MLB rotation by the trade deadline(which means not only his performance but performance of the MLB rotation, Wesneski, Brown, etc), then you build in some break points over May and June to allow for the possibility of him starting postseason games. Otherwise I think you just kinda let him do his thing similar to last year, and consider him as a potential bullpen weapon for the stretch run/postseason if that's deemed prudent.