Growth from 2k to 60k is an important tipping point.

There should be a buyout at this point. Valuation based on income result in 5k * 12 * 15 * 40% = 360k, which should be a fair price. valuation based on funding would be around 400k, which would be similar.

You can have that written down as a contract, not necessarily direct cash payment, but cash payable, and have your co-founder to sign as guarantor, which would not be a bad price for him, and not a bad price for you too. Deduct the 3% if you want.