There is no standard figure. What you add has to pay for your own costs on the job (managing it, the site visits, the risk of putting it right) and leave the profit you want. Decide that margin first, then work out the markup that delivers it.
The trap is adding a round percentage. Add 20% to a subcontractor's price and the 20% is a share of their price, not of yours, so you keep less than 20% of what the client pays.
| Margin you keep | Markup to add | On a £10,000 subcontract, charge the client |
|---|---|---|
| 10% | 11.1% | £11,111.12 |
| 15% | 17.6% | £11,764.71 |
| 20% | 25% | £12,500 |
| 25% | 33.3% | £13,333.34 |
| 30% | 42.9% | £14,285.72 |
| 35% | 53.8% | £15,384.62 |
| 40% | 66.7% | £16,666.67 |
Do it trade by trade. A job can look healthy in total while one trade is priced so tight that a single day's delay turns it into a loss.