Wookieepedia has detailed info on most of the ships in the Star Wars universe. One of the interesting bits of data is the shield and hull ratings. Shields are listed in units of SBDs and hull is rated in units of RUs. I have no idea what these mean, but they can be used to help figure out what hull and shield ratings your custom ships should have in X-Wing.
The following table is based on the values in Wookieepedia (and a few other sources where the wookiees didn't know the value). Like most things in star wars, some sources have conflicting values. Also, it looks like FFG modified some of the ships (TIEs and B-wings for example) for game balance and playability.
Ship shield hull
Using this, you can estimate that you get 1 shield for every 25 SBD and 1 hull for every 10 RU. Shields seem to stick much closer to the 1:25 ratio than Hulls do to the 1:10.
In the end, use your best judgement and pick values that feel right without being too overpowered.
For those interested, the CR-90 Corellian Blockade Runner has about 16 shield and 20 hull. Don't use these values unless you want a long boring game. ![]()