If those numbers don't make you happy, the problem is that your company isn't valued at the speculative 5-10 million that would make what you are asking for reasonable. Assuming the business makes 100k a year, we can guesstimate you want at least 10% of the value of the company for a single year's work. That might be fair or it might not be, but it's clearly disproportionate given the time and risk the others have already put in.
Where does this leave you? In my opinion as a business owner, the best way to get around this is NOT to haggle over the valuation of the company because you have no control over it and neither do your partners. By all means throw the ball in their court and see what they offer, but whatever they come back with remember that you can probably do better by linking your ownership to growth and the amount of new revenue you drive. This will be easier for them to agree to since they'll only give up more ownership if the company performs better thanks to your work. And assuming your work makes the company money - you will be able to push your stake in the business much higher than you would get otherwise as an employee, possibly even up to founder levels.