This really depends on the pattern and how you like to apply the pattern.
Whats simple and often works:
Copy your surface and untrim. Create 2 bounding curves in 2d in front. You ideally take the longer side of your surface, because this often has the greater change.
You divide the curve to create points in between them, then you connect a pair of them with lines.
Project the lines on your untrimmed surface and subdivide these projected curves again. Et voila, there is your Grid. You are now able to modify the borders and you have an almost equal spacing.
Work on your cells, and after you are done, truncate everything at the border of your trimmed surface. If required build some sort of frame.
You can also do this with an 2d surface in front, use isotrim and project the isocurves on your untrimmed shape. There are dozens if not hundreds of variations. But you should never ever rebuild your reference surface if there is no good reason for it!