http://www.w4rp.com/ref/coax.html This is my favorite page for RF attenuation by cable type.
http://www.fab-corp.com/pages.php?pageid=5 shows power drop per a given dB.
RG-8X will drop between 0.5-2.5dB per 100′ and the dB loss scales linearly with the cable length. Worst case, you’re <1dB at the highest frequency for your new HF antenna.
VHF has near double the loss. If you’re running it any more than about 15′, you might consider something thicker like LMR-400. Particularly if you ever plan to use UHF.
A couple weeks ago, my main mast UHF/VHF feed line failed. It was 50′ of RG8-X, which drops 2.35dB @ 150MHz or 41% of the power. That turned my 50W transmission into 30W out the antenna. Factor in the SWR loss and it’s less than that. Upgraded to LMR-400 and I’m getting closer to 40W out. But the main lesson learned here was the root cause of the failure. I didn’t use sealing tape on the connector, and water got in.
I got my cable from MPD. My window feedthrough is the “big one” from MFJ that has 4 UHF, N, F, balanced line, and random wire terminals as well as 12V, ground, and a passthrough port for an antenna tuner or other cable that needs to be snaked through the window. I screwed a terminal strip to the wood on the feedthrough for ground connections from each piece of equipment and drove a copper rod bonded to the rest of the house grounding right outside the window. If you’re concerned with reducing connector loss and noise, go with silver plated connectors and perhaps a gold center pin. If you don’t mind soldering, that will make a better connection to stand the test of time.
Good luck with your antenna projects. If you need a hand with anything, just ask. I’m off work from the 23rd to the 8th, or I could help on weekends. I have a crimper and a stripper you can use.