Hi, missileType for map items is used for Object IDs to use as portals. 584 is the standard red portal, 622 is a grey portal (like the one-time use ones).
Len column is indeed the ID form Levels.txt
I'm pretty confident the calc1 field is no longer used, it used to be somewhat related to map tiers but we have a different system now as the tiers are a bit more involved and there is more hardcode related to them. On that note, the maptier from spellDesc only relates to the displayed tier on the item and does not actually affect the tier itself. That is all just done by updating the relevant fields in Levels.txt / MonStats.txt as well as a bunch of hardcode (like which sigils to drop, stuff like that). Not the most modding-friendly I'll admit.