I was lucky to had some beers measured, and Tinseth come out closest for me, full boils in a converted keg.
Don't get too hung up on it. Find a commercial beer that has a published, believable IBU, like Sierra Nevada Pale Ale, try and brew a similar beer and see how they compare.
Your resolution on IBUs is about 4 or 5 before you can say one is more bitter than the other. The routines require inputs on AA which goes down with time. Process has its own influence. The strain of yeast will have and impact, as the bitter resins stick to the yeast membranes.
As an old tech used to say in the Engineering Lab - "Measure it with a micrometer, mark it with chalk, cut it with an axe". I miss that guy.