"Clark
It'll also depend on how often you race.
I'm in a league where we race twice a week so I have a much longer period where I can develop both this season and next season's car.
Somewhere else on the forum suggested that at the start of the season you should be 100% focus on next year's car until it gets to 50% - as it should quickly go up. Then focus purely on this seasons car. I'm trying that this season and it seems to be working ...
I suspect you and I read the same post. I've been trying this for about a season and it seems to be working, but I use different percentages. Both the leagues I race in run three races per week, so I don't have as much time as you do, but I have more than twice as long as someone who races every day.
This will also be affected by what your competition does. In both my leagues, 70% seems to be about the average at the start of the season, so I try to target that (though I've yet to succeed). Typically that means shifting design to current season somewhere around 50% next season, which tends to vary widely depending on where you start the season with your design.