Usually anything below an SWR of 2:1 is considered acceptable loading (the majority of the power - 66%, is being used up in the antenna system [antenna and feedline]), with the rest of the RF being kicked back/reflected back to the radio not being enough to harm the finals (with the exception of some older solid state radios). This does NOT necessarily mean the antenna is radiating the RF out acceptably ( as I mentioned before, the SWR your radio sees is for the antenna system [antenna, feedline, and [if you have one] antenna tuner/t-match, L-match,etc.], you could have a 1:1 SWR, and your antenna could be lousy radiator if the power is being dissipated in the feedline [G5RVs are notorious for having this issue in certain certain frequency ranges), but your rig won't have a fit (or at the least, SWR foldback protection circuitry, won't massively cut back the radio's output power. The tube finals in tube rigs are usually much more tolerant of mismatches (pre-WW2, hardly any consideration was even given to SWR - people just tuned for maximum RF current into the antenna system, and in many cases, the SWR was well over 5:1, yet tube finals were not used up left and right as a result of this). Most post WW2 tube rigs will easily shrug off a 3:1 SWR. Throw in the Pi-network used to tune/peak the tube finals, and most tube rigs will handle up to 5:1 SWR, since all they need is for the plate current of the finals to be within acceptable operating parameters/electrical load. Some of the tube rigs (like the Johnson Viking II I restored 3 years ago) have pi networks that will even load up (give the finals an acceptable electrical load), at an SWR high as 10:1 (this is due to the fact that when these radios were made, many amateurs did not have SWR meters, so they had to make sure the transmitter's finals, could be adjusted by the Pi-network, to a decent electrical setting [or at the very least, an electrical setting that wouldn't burn up the tubes], despite a lousy antenna system SWR). So basically, acceptable loading, depends upon your radio. Some radios (mostly solid state), are very picky about SWR, whereas some, kind of shrug it off )as long as it isn't extreme (above say 8:1 or 10:1). Adequate laoding typically means that the finals in your rig will see an electrical load/impedance that allows it to send a decent enough amount of power percentage-wise, into your antenna system, and out into (for lack of a better term) the ether.