This is what you need to do and assumes you are familiar with Excel and formulas:
1. Prepare a monthly cashflow for the next 3 years. A simple Excel document that shows what your expected revenue and business expenses are likely to be for each month,with a Total for the year in the last column. If your business is more than a few months old this will be relatively easy. I assume a 7.5% increase in revenue every other year as i don't put my prices up annually. 1 year to 1 worksheet.
2. To each year of the above the above link a simple profit and loss account. Ensure that each cell in the P and L account relates to the Total figure in the cashflow, so when the figures in the cashflow doc are updated each month the P and l self-updates too.
3. Your partnership agreement should state how profits are to be divided - 50/50, 60/40. Using the P and L you'll be able to see how much net profit you are making. In the P and L spreadsheet you need to a) divide the profit between you and the other partner, b) deduct the personal allowance valid for the year in question for each partner c) use a formula to calculate the tax and NI due.
Doing this and you'll have the amount you need to put away - generally 30% of net profit for tax and NI. Mine does not take into account depreciation of capital assets so I am always putting away more than I need. When I work mine out this way I am generally only a few quid out from what the Accountant comes up with.