No template or mitre block will compensate unless you can set the precise angle of the corner - very time consuming anyway. Just cut the mitre at 90 degrees; if the corner is greater 90+ it won't notice anyway but if it's under then just file the back of the cut edges away until the front fits without a gap; or fill in the gap after putting it up!
If you need to find a tradesperson to get your job done, please try our local search below,
or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Please select a service and enter a location to continue...
Are you a trade or supplier? You can create your listing free at DIYnot Local