If I have the tactics and calculation abilities of a 2600 GM but the endgame/opening that of a 1800.

How strong/weak do you think i would be?, what would be my fide rating?, how far does calculation carry someone?

Tactical and calculation abilities of a 2600 should bring you far even if your opening and endgame isn't solid, this is just my opinion though.

You would probably still be 2600 bc you can calculate in the opening and endgame xD

If everything else except the opening and the endgame is 2600 than maybe, this is kind of an odd question. Like #6 is kind of right although endgame and opening understanding can be important at those higher levels if you can calculate and find tactics like a 2600 I don't see why there should be that big of a problem with the opening and endgame.

Maybe 2570 if you know how to play positional chess too. You might lose a few points in the endgame now and then.

on what timeframe? the minute people know you as that master that cant opposition his way out of a wet paper bag, you are prob gonna reach a cap around 2300 tops . 2200-2300 is probably a hard limit. 2600 players,draw vs 2400 and 2500's all the time and draws vs 2300 happen a lot more than say , 1700 vs a 2000. Being really weak in endgames basically means, if a weaker master is not blown away tactically in the first 20-30 moves, it will all go downhill from there. You will especially suffer as black, since white often has the choice to equalize faster so long as dictates the flavor of the game.

But this is a very obtuse chimera you have created. A 2600 is very capable calculator even when tactics is not their specialty. I wonder to what degree would such a theoretical phantom be able to brute force some endgames. My guess? Not as much as people think. There is a reason once you enter that mid GM and above level, enduring ponderous tomes like the 5 volume averbach series on the ending become something like a rite of passage. Even Carlsen has been shown refreshing on Fine's endgame work as reference work. Those rules and examples are necessary to truncate all the general patterns. the people who can calculate ok and know some endgame principles decently well will almost certainly outplay the theory free calculator of ours.

Assume that he has the positional sense of a chimpanzee.