First engineer? I've seen it go as high as 60% especially if the engineer has entrepreneurship or upper management experience. Some can probably negotiate for more, but at some point, the other founders lose interest.

If you're getting a salary either freelance or FT, it's unlikely you'll get more than 10%.

Rule of thumb is that giving equity increases valuation by 1/(1-x). So a 5% share should increase speed/valuation by 5.6%.