fancy conversion from real to unsigned

Its part of the 2008 standard - but you can compile and use the package fine in Quartus. You just import it as part of your project or an external library. Get it from here (these are '93 compliant version of the VHDL - 2008 files) :

http://www.eda.org/fphdl/

Given you had a problem in modelsim, Im guessing there is problem elsewhere. I tried the code you posted exactly as is in modelsim and it worked fine. Are you sure you havent done something like declare the constant you posted in a package then declared it again in the source file, but with the new "wrong" value?