Answering the question requires you to first of all realise that you must be 1000' above the mast if you are IFR.
Um...well not strcitly true - the problem with IFR is that everyone uses IMC and IFR interchangeably. If however you are IFR but flying in VMC clear of cloud in sight of surface below 3000' then you dont need to be >1000' above the highest obstacle. The thing you must realise here is that you do not always have to fly quadrantel rule - ONLY above 3000'.
This is a commonly promoted error.
Simiarly - being IMC does not mean you aare actually flying the a/c by sole reference to instruments - if you fail to have the required separation from cloud etc it is EASY to be IMC yet flying visually - even though at this point you must be IFR.....
So technically the answer is not necessarily as asked - because the assumption now is that you can only fly at the first available flight level above 3000 transition altitude....but that then assumes you arent within the LTMA with a TA of 6000' - Where obviously the lowest available flight level is a lot higher and the mast is irrelevent....
...to be truely correct the answer is mast height + 500' (unless in a congested area + 1500).. turn that into a flight level and whatever that is *is* correct (unless it puts you above 3000' - then you must fly quadrantel) because a flight level is merely a repsentation of a height from the 1013 pressure level. I can fly at FL20 if I so choose.....
Which all highlights the point that the question is severely defficient in the appropriate amount of information and is reliant on assumptions....a very very poorly posed question.
Large a/c on approaches do not take PEC into account - they use radar altimeters below 2500'agl....
Smaller aircraft apply a PEC - typically 50' but on some a/c the CAA will now accept lower values - such as 30' on a PA34.