when its a gamble like this, where you dont have a budget and youre talking about splitting any theoretical profits rather than paying out by hours worked, it gets difficult.
going by ownership of the story, 50/50 is fair. actually, a lot fairer than many artists get - its rare for artists in comics to be recognised as partners. youre collaborators on the story, its a share of each of yours' ip, and so the 50/50 split is reasonable.
if you really wanted to go by labour hours, though, you need to find out how much time your writers putting in. that tends to be harder to quantify as well - writing contains a lot of thinking and figuring that can and can't be counted as 'labour hours' - its on the bus, in the shower, in bed. constantly. but calculate the hours sat down writing, drafting, editing, and researching - plus time spent figuring things out with you, making adjustments, however u do it - and youd get your writers strict labour hours. not a great estimate, but hey. so say they put in 10 hours per chapter (random number) youd then divide ten by 35 (your combined hours) to get their percentage of the revenue.
personally, i dont like that way of working. however, if youre in a situation both where your time is precious, maybe being compensated by relative time spent would be better.
i still say 50/50 is fair though. youre partners on an idea as much as you are coworkers in labour