... also isn’t ideal humidity dependent on the temperature
Guild says "The ideal temperature for solid-wood acoustic guitars is in fact normal room temperature, which is about 70 degrees F (21.1 C). The ideal humidity is about 45% to 55%". In
context, that is relative humidity.
Martin says "C. F. Martin recommends that you keep your instrument’s humidity level between 45-55% and temperature between 72-77".
All other things being constant, as the temperature goes down the relative humidity goes up. So you could make some assumptions about the amount of water vapor needed by the wood, pick a temperature and calculate the relative humidity needed to deliver that amount.
But in practice, this is a case where a strict application of the physics is not going to be especially useful. After all, the ideal humidity for the wood in my guitar will be different than the ideal for the wood in your guitar. So the specs offer both a temperature range and a relative humidity range and if you are in bounds that is good enough.