Your overall goal of adding 25 watts seems reasonable as you haven't been on a program like this for a lot of years and it's likely that if big gains are going to be made they'll be made sooner rather than a couple of years further along this program.
That said, the idea of watts of FTP gained per hour of invested L4 time seems flawed on many levels. As you point out it's not linear and it's definitely not discretely linear as in you might expect a small gain for a small investment. A small investment of L4 time as in a few weeks or a few sessions is likely to produce no gain at all and a very large investment in a short time (e.g. bumping your weekly L4 hours way up) won't force adaptation faster than your body is capable of adapting.
But a steady investment for a long enough period may well give you the gains you're after or perhaps more. Trouble is, no one can estimate your potential to improve or adapt. The only way to find out how far you can take it is to invest the time and energy and see what happens. There aren't any shortcuts or reliable crystal balls you just have to do the work, stick with it for the long haul and adjust your program as necessary over time.
FWIW, two thoughts on your current program:
- Ideally you'd do specific bike training on more days per week than you're not training on the bike. The running will certainly help as in it's a lot better than complete rest and cross training in general works best for folks with less sports-specific training and doesn't work as well as your cycling fitness increases and on the bike specificity becomes more important. But if cycling and FTP improvement is your primary goal I'd increase the number of days on the bike and set them up so they're more or less evenly spread throughout the week so don't cluster them all together. Basically it pays to maintain reasonable sports specific training frequency and it pays to train on more days in a sports specific way than you detrain even if some of the days need to be shorter and or easier.
- I'm a big fan of longer L4 sessions like the 1x45 work you're doing but I'd still mix it up a bit with some steadier 1x45 style days and some more conventional 2x20 style days where the intensity is a bit higher. Other options include things like 1x45 or 1x30 or even 1x60 days with work like micro-burst intervals. Lot's of flavors on these but the main reason to do them for a time trialist is just to mix things up and make the indoor work more interesting and tolerable. An example would be say a 1x30 Tempo with a Twist workout, set a base pace in comfortable mid Tempo (e.g. 215 watts), ride that for 2 minutes, then whip your legs up to burst for five to ten seconds before settling back into base pacing till the next 2 minute marker rolls around. Overall AP will likely end up in the general range of high Tempo/SST which has a lot of direct FTP building benefit but the bursts are both good for adding some dynamics but mostly in this case for breaking things up mentally and giving you a goal every couple of minutes. Fifteen of these quick bursts and the half hour is finished. Lot's of variations on the theme or things like over/unders can be used to mix things up and make them more interesting but if you hate the trainer (and most of us do) then a steady diet of 1x45 L4 work sounds like a training burnout waiting to happen.
Good luck,
-Dave
