Your goal is laudable. However, it is flawed by incorrect terminology.
Whether the IEFBR14 program as used in JCL Basics/Allocating Data Sets is a "utiility" program is debatable. In my opinion "utility" programs do something; the IEFBR14 program basically does nothing. This characteristic can cause something like this to not work the way you expect.
//A EXEC PGM=IEFBR14
//ATAPEDS DD DISP=(NEW,CATLG),UNIT=TAPE,DSN=data set name
will not create the data set and will not catalog the data set. The program must
write something to the data set for the system to actually catalog a data set on magnetic tape.
"Extend" as used in your text is incorrect. The word is a verb that usually means "to add to." The word you want to use, though it is also incorrect, is "extent." "Extent" as used in this context in incorrect because the word specifies a space on the disk. An "extent" might mean something like cylinder 33, track 0 through cylinder 33, track 2, 3 tracks in total.
When you specify something like SPACE=(TRK,(35,3)), properly speaking you are requesting an "initial" or "primary" allocation of 35 tracks, and a secondary allocation of 3 tracks that will be used if your program fills the initial 35 tracks. Just to add to the confusion, this initial allocation can be divided into a maximum of 5 actual extents, but it will be a total of 35 tracks. Unfortunately, this can confuse some programs. Say the initial allocation contains 2 extents, 30 tracks and 5 tracks. The ISPF data set attribute display will incorrectly claim the primary allocation is 30 tracks because ISPF thinks the first extent is the primary allocation. ISPF has no way of knowing the primary allocation was 35 tracks because it is not a data set attribute.
Another problem relates to what is stored. SPACE=(4096,(350,30)) is equivalent to SPACE=(TRK,(35,3)). ISPF will see the 35 track first extent, but will show 350 4096 byte records because it will see the secondary allocation is 30 4096 byte records and extend this to the first extent by calculating that one track can contain 10 4096 byte records and multiplying 35 tracks by 10 4096 records per track.