Just enter the amount of equity you still had in the business at the time of selling your share. That includes your share of the profits of the final year which were not distributed.
Look at the beginning capital account of the final return, add any contributions, deduct the distributions and add your share of the profits during the year (or deduct your share of the losses). The only reason I laid it out like this is, because you might have a zero ending capital account balance on your K1. Normally you would just take the ending capital. This is not a valuation by any means, but it will be good enough in recording the gain or loss on the sale.
