Don't over complicate things. Here's what we do......
Across the North Atlantic - 100% Cattle class, except for medical reasons.
All other routes:
Total door -door time>24 hours = business class
Single flight leg >9 hours = business class
Everything else cattle class, unless escorting a Client or travelling with a Client & they're paying.
Use of LoCo's is preferred, but not mandatory. With a LoCo, We always book 1 bag as hold baggage, as we can never guarantee not having to hand carry parts, tools, documents or other hardware between plants, offices & sites. You might go out with just a laptop, but you can often end up coming back with a transit case & it's often cheaper (& quicker) than sending the stuff by courier!
Tickets are booked as changeable for a nominal fee as a minimum if fully refundable are too expensive or not available, except of course for LoCo - they're throwaway.
We get to plan our own itinerary to suit the times/dates/places we need to be, but it's booked through a travel agent & they keep us honest on the route & the cost [and Vice Versa!] & the boss signs off on our travel plans so we don't get too extravagant.
Personal use of air miles / frequent flyer membership is permitted , in fact it's positively encouraged. As a company, we work on maintaining relationships & enciuraging high quality service, so I guess it makes sense for us to build up a picture of routes and carriers we can rely on & take advantage of it.
Hope this helps.