A big challenge you will face in bringing a partner in is to identify the value of your firm. There are a number of different ways to do this, one of the more popular ways being to discount future cash flows for a determined number of years (typically 5). You are discounting future cash flows to present value. You can then multiply this amount by the percentage of the firm your partner is purchasing. I am not sure how big your firm is, but typically people hire an accounting or consulting firm to perform such due diligence. However, if your firm is not that big, cost-benefit will come into play because having such work done can get pricey.