Actually it's 100% to spawn a bloatfly on death. Bloatfly variation depend on zone's level cap and player's level. Legendary Bloatfly chance depend on zone's level cap, player's level and difficulty setting.
@The Appalachian
A few garbage I didn't bother to screenshot but Legendary Glowing Bloatflies can drop high tier legendary such as fatman and plasma guns. This means Home Plate can be used to farm any legendary item on any difficulty. In fact survival is recommended due to increased legendary spawn rate. ( outside of survival you should just console command legendary you want anyway )
( actually this is Breakheart Banks but you get the idea, those Legendary Glowing Bloatflies can drop good staff )
Bascially you can sleep to save, toss a relay grenade to summon a synth, syringer it and kill it to spawn a bloatfly, if it's legendary kill the fly to check the loot, if not load the save.
And if it's legendary and have a good drop, you can save immediately due to a bed nearby.
This means any location with sleeping bag/bed/mattress can be used to farm legendary items.
The only downside is relay grenades can only be purchased in bulk once you beat the main game with the institute and that's the end of the game already excluding DLC.
Now I'm been testing using the minuteman spawned by flare gun to farm legendary.
Another way that works is using recruit beacon to spawn settlers to farm.
Nuka Town USA also has quite a lot of disposable NPCs can be used to spawn Bloatflies. ( keep one raider alive or Amoral Combat might break )
This way the syringer farming method can be used a lot earlier.
Bloatfly Gland in FO4 is basically lottery ticket.