Ah! there lies the problem. Unless you have tons of time and tons of enviromental information to hand it is pretty much guess work. There are so many variables. Wave period is probably the biggest consideration as the wavelength [hence period] and height are related in the open sea, but there are always smaller waves superimposed on top of the bigger ones to confuse the issue and then you have the relfection and splashback from other risers and the jacket to consider.

Taking a 'dumb' DLF of two may seem a bit of a cop out, but the superposition of different modes can throw up some strange behaviour. I remember seeing one offshore wellhead that danced around vigourously for ages, but every now and again would 'go mad' for a few seconds. Whether it was slugs of water or sand coming up or some combination of modes coming together I don't know. I've never had the time, resources or mentors to hand to chase this thing through fully.

You could try to define a response spectra or similar, but it is something I have never done [or seen done]. Generally be a bit more cautious around isolated weights like drain valves or small bore [ie low stiffness] lines with valving, where there is little to stop them 'bouncing'and they are the 'tail being wagged by the dog'. The main flowline may not need a high DLF if the conductors are chocked down to reasonable gaps.
Personally we do the fatigue calcs outside of Caesar in here and we apply the DLF directly to the stresses being considered as part of the fatigue calc.

Conservatism in this area is good. The cyclic endurance required of these lines is mindblowing; ~1E9 cycles upwards with movements of +/- 3" is not small beer and the consequences of a failure from fatigue or otherwise does not bear thinking about. Nobody ever got fired for not making a platform go on fire.