Just to add, if you did trust your signature you could look at your power curve which should have a number for 5 hours or more.
Then it depends on the course profile - as you know from doing it before, if hilly (and especially if technical or very steep descents) averaging 200w will mean going quite a bit above that on the climbs due to low / zero power on the way down. Not sure Xert can help there actually
So, I would actually just look at how much higher your LTP (or 85% TP as a proxy) is vs last time you did it, and add that difference to the power you put out on the flats and each climb last time…? Probably even more for the climbs and a bit less on the flats in an absolute sense. So maybe doing it % based is better - if TP 10% higher, target 10% higher power throughout… Assumes you paced reasonably last time / didn’t bonk of course