(Below and up to Level 5, each +/- 1 magnitude = +/- 1 Level. After Level 5, each 1 magnitude = 5 levels. For items, base R/D/T adjustments (in Magnitudes) happen before Level adjustments for things like #/day, etc.)
Spell level increases for range, duration or target increase the spell one step up on the chart for spell level which generally goes:
1-2-3-4-5-10-15-20-25-30 etc.......
So look at your base and then as you add an increase, jump it the next step. so base 1 +1 touch means 1 goes to 2. Then add moon which is +3 so 2 goes up three levels to 5 on that line and then adding room for greenhouse or courtyard is 2 more which causes 5 to go 10 then 15 for total of 15. IF your base is a funny number (like 17 which you can have for general spells), then look at the nearest number above it and jump accordingly (17 goes to 22-27-32-37 etc).
Magnitude is spell level/5 so once you reach level 5, each +1 equals a magnitude increase.
Again why would anyone do that?
The base 1 effect already covers 10*10 paces or the equal of a large tree.
Takes a while to get used to it. And its horribly easy to do typo´s and brain crunchies alike.
Well, since you always ask so nicely then you deserve to have it explained to you: You see, DW, it might be because that base effect is just for a Target of "Individual", and someone might want a greenhouse or garden with more than one plant or tree. Just as a wild guess...
I guess that "math" stuff does get pretty hard after the number "one" sometimes, huh?
(Or, again, is that another typo there?) 
thanks for the clarification folks. I'll have to recaculate some spell levels in my home game, storyguide permitting.
If the spell is off, it ~should~ be rebuilt correctly, just so you and the SG and everyone else has a correct "feel" for the magnitudes of spells and how powerful they are at that magnitude, and likewise for roughly what magnitude spells a mage of X experience might be wielding.
If it's a question of your mage character having achieved ~significantly~ more powerful effect than s/he should have, and formulaic effects that have been used in-game, then you have to balance that handwaving that back-history against any adjustments. At worst, throw yourself on the mercy of the Troupe. 
Come up with similar effects at less range, or less duration. Or just bite the bullet and re-build the character - some spells simply aren't the same with significantly lower parameters, and if every spell is castrated, it makes no sense to have them all be next to useless.
But that mistake is not uncommon, quite the opposite I'd say. It's a learning curve we all went through - but once you've got it (and it sounds like you now do), it becomes easy.
We have cheated our worldy goods with a circle that matures plants in one sun. This makes for timber for our constructions and grain and other goods for our food. We have not yet started to capitalize riches, but have given the local lord a lot of wood and food to help him capture Rijnsburg so we can destroy the church there that is eating away the aura's.