I think you need a dummy load.. if it's a guitar amp I heard some were equipped with the ability to drive another speaker (tweeter etc) and if it's an amp head it's designed specifically for this purpose.
Either way connecting a "powered" output into a line level device will break it.
If it's a tube guitar/bass amp I'd mic it if I were you but some prefer DI signal for reamping, although you get some tube sound from the amp circuitry with the dummy load.
For "real" DI you'd run the guitar through a box that splits the signal in two; the other comes straight from the pickups and the other goes to the amp which is miked and you'd multitrack and mix/reamp them in post (many newer transistor amps have a DI/thru output for this purpose)
The use of DI in recording is said to be useful if a multitrack is archived and remixed decades later as the sound is recorded "as is" without colouring it with amps, FX etc which makes it easier to reamp (re-record it from the desk/recorder through another amplifier)
As for the formulae for the R value it's hard to tell but a dummy load isn't much else than a power resistor in series, like 5W, your best bet is to test a few values (I guess line level in this context means 1Vrms max) but many consider resistors generating noise in audio circuits due to heat dissipation.
A "technical" term for dummy load is ballast resistor which is often used with LEDs, say you have a device with 24V power supply and want a 5V power on-LED so you'd put 1MOhm resistor in series first and downsize from there, ending up probably somewhere around 10-100kOhm.
To stay safe you could use two, one in + and other in - but someone more experienced could confirm this (as diodes such as LEDs don't need the resistor in minus because they're polarized and operate with DC)
Some power amplifiers have ground lifts which means switching off the minus but their use is seen as questionable (I've seen one actually, and afaik some DI boxes have ground lifts, I've never owned one though)
In theory adding one shunt (from + to -) acts as a dummy load too, in power supplies they're called load resistors (I once tested this with a diode bridge into a cap but I can't vouch for it)