You are going the right directions. I just checked my older 46th addition of the Lyman Manual and it also recommends a 40.0 starting load of IMR4320. And you are correct that there are a number of different recommendations. But commonly the recommendation and you are following that is starting low and work up. Starting low takes into the consideration in the differences in chambers, barrels, powder and primer lots, and bullet design.
As an example I had been loading Norma & Hornady150 grain bullets for my .270. I decided to give 150 grain Sierra Boattails a try, I was hoping to work up to the same hunting charge I was using for the other bullets. But as I approached that charge I started seeing signs of unacceptable pressure and drop back.
I have three different additions of the Lyman Manual. I have found it to be a good reliable resource. But I also own other Manuals.
Following the link to additional data.
http://data.hodgdon.com/main_menu.asp180 GR. SPR SP 180 H414 .308" 2.800" 46.0 2433 39,800 CUP 49.0 2573 47,500 CUP
180 GR. SPR SP 180 Varget .308" 2.800" 41.0 2470 41,200 CUP 45.0C 2661 49,600 CUP
180 GR. SPR SP 180 BL-C(2) .308" 2.800" 42.0 2460 40,300 CUP 46.0 2660 50,100 CUP
180 GR. SPR SP 180 H335 .308" 2.800" 38.0 2374 41,100 CUP 41.0 2528 49,500 CUP
180 GR. SPR SP 180 H4895 .308" 2.800" 40.0 2454 41,200 CUP 42.5 2595 49,700 CUP
180 GR. SPR SP 180 Benchmark .308" 2.800" 38.0 2363 40,700 CUP 41.3 2542 50,800 CUP
180 GR. SPR SP 180 IMR 4007 SSC .308" 2.800" 45.0 2481 46,600 PSI 48.0C 2616 54,200 PSI
180 GR. SPR SP 180 IMR 4320 .308" 2.800" 41.0 2407 43,500 PSI 45.4C 2665 57,900 PSI
180 GR. SPR SP 180 IMR 4064 .308" 2.800" 40.7 2445 44,100 PSI 45.2C 2683 58,200 PSI
180 GR. SPR SP 180 IMR 4895 .308" 2.800" 40.5 2439 43,800 PSI 44.7C 2674 58,700 PSI
180 GR. SPR SP 180 IMR 3031 .308" 2.800" 37.0 2372 43,300 PSI 40.6 2594 58,000 PSI