Hmm. Personally, I like the break at level 5.
Yes, I agree, it does feel arbitrary and "hack"-ish but it forces an early career decision for the character in that the player has to decide whether or not the character wants some big magic early and therefore must specialize in an area of interest, or wants lots of low-level ('scuse me, 'low-maginitude') flexibility leaving moderately large magic for much later in thier career.
I've played a bit with this and I'm finding the current RAW very well thought out indeed. Yep, it does seem hack-ish. But in the physical science world where I have spent a lot of time it feels right nonetheless -- structural and environmental subtleties are not terribly important until you start talking about "large" or "fast" or "hard" or, gawd help you, all three. Then you have to be REALLY skilled if you want to push a large plane (elasticity 'break points') past Mach 1.0 (structural 'break point') in a fuel-efficient manner (physical Limit). Almost anyone can toss a paper airplane across thier living room and get it to land with acceptable damage, even with a small cargo ... a small chocolate, perhaps.
But, can a mechanic be devised that takes out the hard break at level 5. Sure. Off the top of my head, consider this:
-- Leave the Base Guidelines intact.
-- Keep the "magnitude cost" of each RDT as it is.
Start with a base spell, say, create an illusion that affects three senses (base level 3)
CURRENT SPELL LEVEL 3
Now, for each magnitude of modification called for by the rules, iteratively increase the spell level by, say, 25%, rounding up to the next whole number. So, if we want it at voice range (+2 magnitudes):
1st magnitude modification: 3 + (3 x 25%) = 3.75 round up to level 4
2nd mag modification: 4 + (4 x 25%) = 5.00 (exactly, no no rounding)
CURRENT SPELL LEVEL 5
Next we want it at Sun duration (+2 mags):
3rd mag modification: 5 + (5 x 25%) = 6.25 round up to level 7
4th mad modification: 7 + (7 x 25%) = 8.75 round up to level 9
CURRENT LEVEL 9
Keeping our target at individual (+0 mags) means we have a final spell level of 9, something that anyone with a CrIm casting total of 15 + stress die could spont most of the time. Using the RAW would make the spell level 15 and require a casting total of 27 + stess die to spont at the same frequency.
Increasing the percentage-per-magnitude modification would make specialization more rewarding, decreasing it makes the generalist more capable.
Note that if you start with a high Base level, things get very difficult very fast. Start with a Base guideline level of, say, 20 and do the same four steps. Final level with house rule: 50. Final with RAW: 40.
If a person chooses a straight 3-levels-per-magnitude approach, they really rip the guts out of the idea of "small spells" and wouldn't able to do much at all as a young generalist magus. Though, the really powerful stuff does become somewhat easier.
Starting base 1 --> 4 3-level increases --> final level 13 (RAW: 5)
Starting base 3 --> 4 3-level increases --> final level 15 (RAW: 15)
Starting base 20 --> 4 3-level increases --> final level 32 (RAW: 40)
Or, one could spend time modding all the guidelines.
Clear as mud? <<boy, that was a lot more than I intended to write>>
Best,
-K!