There is no such thing as a standard calculation for commission. But there are some standard guidelines and rules to be followed.
Essentially you need to create a win-win for your company and your salesman. If he/she is to work full-time, then he needs to earn a fair full-time income, and of course, a pro rated share of this if working part-time.
Plus, if he is to pay his own expenses, that needs to be built into the calculation as well so the commission earned fairly allows expense reimbursement.
On the other hand, if the salesman is a 'multi-line rep' and is going to clients and representing half a dozen different companies, then clearly, the amount you would expect to pay is reduced.
So, how much income do you expect this person to generate for you, and what dollar value of ongoing revenue will they manage? Match that up alongside a target earnings, and calculate the percentage commission from that.
Don't try and cheat. If you make the commission too low, you'll lose the salesman after a short while. On the other hand, if you make it too high, you'll regret it yourself after a while!
Generally it is best to pay commission on the gross, because the salesman has no control over expenses and other things, except in cases where they know that they have a product costing, eg, $100, and they are free to set its selling price - $150, $200, or whatever else.
As for a split between landing a client and ongoing client maintenance, it depends how much of the client maintenance comes inhouse and you'll be doing compared to how much the salesman will be doing. Do you really want your 'new business salesman' ending up just maintaining existing accounts? Don't you want them pulling in new business? Or are your clients always 'at risk' of being poached and do you need special focus to keep them?
Sales people are often lazy, and what sales person wouldn't prefer an ongoing revenue stream for doing little or nothing with existing customers! You want to structure it so that most of the income is from new business.