I have also yet to encounter such tool. I guess it has yet to be developed. Probably because there's less demand for such kind of tool. All the Bitcoin calculators that I've encountered are based on Bitcoin's real time price.
To @OP:
what you're looking for is something simple to make yourself by using a spreadsheet. You would just need to manually enter the Bitcoin value and have the result calculated automatically
Thanks I was hoping that a service already existed but I can do it manually with a spreadsheet. I have been doing it manually without a spreadsheet which is hard work.
The spreadsheet won't automatically update as what you are looking for, it also wastes your time keep updating your portfolio instead of having a research and it might be this tool that you are looking for,
Blockfiolio app, or website. You can trace and track your coin at an accurate rate through any exchanges that you want and it has already given what currency value you need to know.
I guess OP is looking for a specific tool which answers the question, "how much would my Bitcoin be worth if its price is $______?" So the input would be OP's total Bitcoin holdings and the speculative price of Bitcoin. Blockfolio does not provide that. Just like the rest, it provides real time Bitcoin price.
If OP really wants to keep track, spreadsheet is his/her best option. There is automatic update or computation with spreadsheet as far as his/her worth is concerned based on a specifically determined Bitcoin's price, not the real time price.
Or are we just making things complicated? Why not just use an ordinary calculator? It takes only a few seconds.