Brilliant question
@Colm. At least I think it is brilliant because it is the sort of question that you might think ought to have a precise answer but in reality doesn't.
As I have suggested before, as far as I could see, the carb content of anything is a bit of a guess. Its even worse when it comes to the humble spud (an example I have used before) where there are lots of varieties, grown under different conditions, in different countries with different climatic conditions and eaten after different periods of storage under different conditions after being cooked by any one of a number of different methods. All those things will have some effect on carb content and to make any sense of the carb content quoted you need to know all about the history of the spud in question and that is never given. Don't be taken in by the apparent precision in labels on packaging. A good guess is allowed in the regulations.
All you can say about spuds is that they are in the higher range when it comes to carb content. Does not matter whether they are whites, reds, roosters or whatever, whether they are boiled, roast, fried, au gratin, dauphinoise, or whatever, they will have a carb content that needs to be taken into account if you are a T1 requiring to bolus for a meal or aT2 using carb control to keep blood glucose under control.
You can weigh things out and look up values in tables and do complicated maths to figure out the what carb content might be under a given combination of circumstances but whatever the answer you get will be subject to quite a big error.
I say do not despair! The best way of dealing with spuds and other foods in the high carb range is to get good at knowing what the effect of those foods are on you such that you avoid some things and limit portions of others semi-automatically.
T1's generally as far as I can see eventually become very good at doing this, eyeballing a plate of food and judging their bolus with confidence. That just comes with experience after a few years of trial and error guided by some general assumptions.
It is where blood glucose monitoring comes into its own for a T2 - you can quickly figure out what things are problems and what are not. I very much doubt if you took a decent sized rooster, baked it, boiled it, roast it or chipped it you would see a consistent difference between the blood glucose rises from the different cooking methods.
The answer is nearly always in the numbers but you need an appreciation of the fuzzyness of the numbers to use them to good effect and not get lost in the fog.