Newbie question

I am sure this has been disscussed before but i will try telling it in my situation.
I was offered a job with a company once i get my class B CDL which i now have my learners permit. The company has told me i can use there truck to take the test in (its a peterbuilt 26ft flatbed tandem with 3rd axle)
the truck has a 3406 cat in it and also a 15spd eaton.
I have tested the truck once so far before i have to take my driving test.
The truck isnt a problem to drive..doesnt feel to large and upshifts very easy...my problem is of course slowing down. (am sure some will tell me what i have done wrong)
coming off exit ramp from interstate trying to slow down i was in i belive 9th gear..well i go to downshift..using clutch and i cant get 8th...i tend to have that issue alot and usually have to resort to getting way way slower and almost just starting back al over again.
what can i do so that i can figure out when this truck hits a good shift point. I have drove normal everyday stick shift for many many yrs but seems like these trucks are a new ball game. The guy riding with me told me slow down pull it out of gear hold it in front of next gear with slight pressure and rev engine..it would fall into gear...which it will, when i tried it..but he is saying dont use clutch for that...but i have to use clutch for the driving test.
whats some suggestions i would appreciate any type of help.

Or maybe some owner operators out there near knoxville, tn that wouldnt mind giving some pointers

 

Before being offered this job i was a diesel Mechanic at a rock quarry so i had to work on there road trucks alot..at that time i didnt have no cdl permit or nothing so what time i did drive was on the quarry which wasnt much speed or nothing..not even enuff to have to go to hi side then.

The truck i drove yes does shift very smooth...actually caught myself few times not using the clutch to shift it.
I dunno if this sounds right to you all but the guy riding with me said that truck likes to be tacked out then shift.
But the trucks at the quarry liked around 1500 to shift..they were internationals.
I dont have an issue geting pulled out or stalling, that truck pulls out nicely in 3rd..My problem is like i said when coming off interstate or such and having to grab a lower gear all while still making myself slower for the intersection.

 

Thanks i do appreciate that, I was told that the tranny and clutch was replaced not long ago..and also come to find out this used to be a daycab peterbuilt road tractor and it was converted to the flatbed it is now.

 

I belive my biggest problem was not slowing down enuff and running out of ramp and was still on the hi side of the tranny.
I was told i could come down to the yard when the truck will be there again and test it out some more...isnt a real big area but should allow for maybe 4 gears...
I belive what got me the most is when i drove it thats first time i have ever had to flip over to hi side and then also back to low side..knowing when to hit the hi side wasnt a problem just trying to find that sweet spot to hit back to the low side is what i am going to have to focus on alot.

 

Oh i am sure you wont have a problem adjusting your seat and such i have worked with a guy once who was very short...maybe 5'3 so i dont belive you will have much problems in adjusting your seat.

 

I am hoping TN isn't that bad on if you miss a gear.
Wish i could get my hands on an automatic or something else to take my test in at least but its not looking to promising....could always rent a truck from Ryder or such to test in but there hard to come by there CDL trucks. Unfortunately i wont have many more runs with the truck to practice in as it is a very much so used truck...so i might have only 1 day or so left to practice in it.