It would be helpful to know the type of startup, the stage of your first product, how much money you are looking for, and all of the data an investor would need to have to evaluate a potential investment, of any sort.
I will assume that you are seed stage and have no idea how much money you will need. Supplement your information if these are incorrect.
First, do not start by looking for investment but look for all available government, institutional and corporate grants, and partnerships with corporations who may have an interest in the technology or in selling the final product through their channels. The former group will likely not look for equity or seek only some warrants and the latter group will probably ask for some equity but it will usually be smaller than a typical financial investor.
Second, if you do need investment, aside from the range, a successful investment by a professional will usually require giving up about 30% of the company. You can calculate it all different ways you want, you can use any formulas, but a professional deal will usually be about 30%.
But you will find few professionals willing to do a seed round with a product that appears to be not ready. Your best bet may be angels and you may be able to do a deal for less equity with angels.
And don't forget Arthur's comment just earlier. You can also try to reframe the deal. Arthur suggested taking money in return for a share of the revenue which is one viable alternative. Many high roller angel investors in fact tend to prefer this these days. An adjunct to that is that you can combine that with a cap in dollar terms at which time the cash stream as a percent of revenue converts to equity at a preset rate.
Because I think you are at a very early stage with commensurate higher uncertainty than even the normal startup, think in terms of changing the shape of the deal, whether in the direction that Arthur suggests or in other ways that are different than straight equity.