Sing
Your numbers look correct. 3.2 deg = 339'/nm.
Don't forget that normally you should allow the 50' TCH when calculating. However not sure if Jepps have factored this in to their table without looking but I suspect not.
To find the DP for a 3deg slope divide the height (-TCH) by 320 which will give the nm to the threshold. To be super accurate use 318'/nm!!!
Also don't forget to add 0.3nm or so to permit the a/c to actually begin descent.