YC puts roughly $20k for 8-10% giving a rough valuation of $200k-$250k.
I'll leave you to your own devices to figure out whether those percentages sound good.
To decrease the %, you would have to convince them to value your company for more than the ~$225k range they appear to have settled on.
You can negotiate and ask for a larger valuation, but, you risk losing the investment. Once you receive the investment, what happens to your risk? You get a salary for 6-12 months and they potentially could get $0 at the end.