In general that is fine. Keep in mind that different types of snakes have varying needs.
For example, a 6ft boa is more snake than a 6ft carpet python, which is much more snake than a 6ft kingsnake (if there exists a 6ft kingsnake).
And, a boa is content to not move around alot, whereas a carpet python is a bit more energetic. A carpet will take advantage of a very tall cage with overhead heat up high above a branch for example.
Formulas are fine as a guideline, but I think the best approach is always to get as big a cage as space and money permit and taylor it to the animal being kept, rather than say (in context), "Everyone gets 4ft x 30inches because the calculation came out that way."

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Mark