Yes practice make perfect but when you can't practice in the wild and you don't know if you will scare the moose away or bring it in. It is nice to have a back up plan.
Bulls are not that fussy just make sure you have consistentcy...
I also find "no-wind" conditions always play the hugest factor.
More important than calling is making sure there is fresh Bull sign in area, look for fresh raked or busted trees, tracks, rut pits.
When Bulls are in heat of the rut they will be calling(grunting on their own) even IF they do not have a cow.
They are not fussy when coming to a call.
Use a cow call with eee-aaa---rrrrrr-nnnnn sound with nostrils pinched, tongue rolled back and thumbs on side of your throat.
Just about any material can be used as megaphone to amplify the call.
Keep repeating your calling sequence every 15 minutes.....it helps to be near water so you can add some other sounds moose love.
I once called a Bull Moose right through our lakeside camp by getting water at lake for morning wash water...the sound of water splashing while I filled a pail brought a Moose right through camp at 5 AM lol.
Spend time at night listening in dark and you can often hear moose calling cows Bulls and moose mating as well which is cool to hear. I will often bring a bicycle to camp and ride logging roads at 4 a.m....stopping to listen.
Mimic what he does....when he gets closer he may rake a tree ...do the same...try not to be too aggressive just let him know their is another suitor vying for the cow.
Sometimes raking alone will bring in a Bull.
More times than not a Cow will be calling, sometimes agitated and very loud, because she is being harrasssed by an immature bull and wants a more mature bull to mate her(mother nature's instinct). So if an approaching Bull does alot of tree raking and does not get a response of a Bull he will sometimes become silent...sometimes leaving quietly or circling quietly downwind. He becomes suspicious at the lack of response to his challenge....often some tree raking will seal the deal and the Bull will come in on a string, glucking the whole way...