As Nigel M and NickJ have said, not too loud as that can have the effect of having smaller beasts and bigger beasts also to stay away.
Don't overdo it either and be sure and wait for a decent time between whistling, relatively simple to see sika in open slightly scrubby birchy ground but the thick Sitka plantations with very few deer lawns and rides are a different game altogether.
I have watched a whistling stag from a knoll vantage point ( wind in my favour for the best part of the ground I'm looking on) with a thermal and he would be half a dozen trees in from the edge of the plantation. You give a whistle and he is on the move working along in the cover, slowly working around and around until he either gets your scent or is at the edge of the trees and will not come further.
Other times you can see them on open ground standing about hundreds of yards distant and you give a call and

he is turned in to a frenzied beast with heather, moss and scrub flying up in the air as he swishes about with his antlers (not a happy chappy

) and he will more often than not come in, sometimes slowly with a head down swinging from left to right swaying like one I had last year and then again he might cover 100s of yards in quick time so always be ready and if you think he is going to run over you, you had better get him shot quick, or it may be close quarters stuff.
A good friend who taught me a fair bit about them was like the pied piper, we would slowly walk in to a bit amongst or beside some right thickety and windblown Sh!te and he would call quietly and finish with a few hind mews and just like magic a staggie would appear. You just have to be brave and give it a go.
There will be days when you get no reaction at all, as they will go at it hammer and tongs for a few days and then switch off for a few days.
Either way they are ghost like at times and I have the utmost respect for them and pound for pound the sika come out tops. They are very hard on each other as you will see with broken, splintered tines and antlers snapped off when you get a mature beast it very often has damaged tines.
Cheers