Myself, just based it on my use and the LA battery ratings. Even our longest day on the water, the LA batteries were still above the critical 50% charged level, when we got home. Ours is mainly using anchor lock in light to heavy current, and some location adjustment, and slow casting tracks. I imagine trolling is what most of your aH are used on. With LiFePO4's, you can go well below the LA 50% without hurting the battery. So if you were good with a 100aH LA, in general, a 100aH LiFePO4 will be more than enough. I'd say that a LiFePO4 that is 75% of your LA ah rating, would be suffcient. Some say even 50%, but that was too near the margins for me.
Now, if you have the room, going larger will never bite you.
