Hello vinylshadow,
I believe most set their cutoff frequency according to their main speakers bass extension capabilities and personal preference discovered via trial and error experimentation.
My opinion is that 80 Hz is too high since that's the generally accepted borderline between bass being directional or nondirectional. You can't tell where the bass is coming from below about 80 Hz but can begin to tell above this frequency.
The goal is to integrate the bass from 1 or more subs as seamlessly as possible with one's main speakers. The subs should remain completely silent until the source recording contains bass content at or below the low pass crossover frequency setting. The first step, which you've already done, is to find out the rated bass extension of your main speakers, which you stated is 35 Hz.
By coincidence, my main speakers (Magnepan 3.7i)also have a rated bass extension of 35 Hz but I utilize an Audio Kinesis Debra 4-sub distributed bass array (DBA) system to supplement the bass performance. My point is that your optimum crossover frequency is probably going to be a bit different than my setting (currently set at between 40 and 50 Hz) for seamless integration. But I'll describe the procedure I used in the hope that it'll work similarly as well for your single sub as it has for my 4 subs:
1. Optimally locate your sub in the room using the "sub crawl" method (you can google it).
2. Play some music with good and repetitive bass content.
3. Sit at your listening position, set the crossover frequency control on your sub to 60 Hz, set the phase control to"0". Then have an assistant slowly turn up the volume control on the sub until the volume of the bass from the sub matches and blends with the volume of your main speakers as closely as possible.
4. Still seated at your LP, have an assistant slowly lower the sub's crossover frequency as low as possible with the bass still sounding very good to you ( "very good" meaning the bass remains solid, powerful, detailed, dynamic, seamlessly blended with the main speakers and natural sounding). Remember, the goal is to not even hear the bass from the sub until the source material calls for it.
5. Still seated at your LP, have an assistant slowly increase the sub's phase control upward from the "0" position to the "180" position until the bass sounds best to you ("best" meaning loudest, most detailed. best blended with the main speakers and most natural). Note that these changes in phase settings will be subtle and you may need to closely listen through several slow "0" to "180" cycles to discover the optimum setting. Just do your best since this setting isn't usually critical.
Hope this helped,
Tim