Max Volume

What's the max volume you'll do with a 5 or 6.5 gal bucket with a 1/4" ID blowoff tube?
I've typically read to reduce volume by ~1.5 gal, and I sometimes get a mild explosion through the airlock anyway.
I'm replacing my 1/4" ID siphon tube with a 1/2" for bottling, and so I plan on using the tubing for blowoff.

If you have a carboy i would get a big 1 inch tube if you plan on going within a half gallon-gallon.I think the 6-6.5 carboys are meant for 5 gallon primarys so thats about right.I tend to get within 3-4 inches from the top of my wide mouth 2 gallon glass fermenters,i keep temps cool which seems to tame the krausen a bit but if i max out too much i just put a blow off tube on for the first week.Cooler ferments give you time to decide whether or not you need a blow off because it generally moves slower which you can decide if you need one or not by observing it periodically,nothing wrong with using one all the time though.Iv'e had some quick and violent ferments in mid-high 60's before so luckily i had a blowoff otherwise krausen would have been splattered all over,i still had a puddle even using a blowoff and jar-thats when you need a blow off for your blowoff jar.

No carboys (though I just have to get one to see what's goin' on in there ) at the moment.
I typically keep my swamp cooler at ~ 63-66* ( I don't have a fermometer). I'm assuming that the inside temp wouldn't (couldn't) get much higher than a couple of *s higher than that.
So I ought to stick with the 1.5 gal head space regardless? For my 5 gal I'm now giving it 1.2 gal head space just because 3.5 gal - grub isn't a whole lot.

Would it matter if I upped my volume to 4 - 4.3 gal in a 5 gal bucket?

No, i do 1.7 for a 2 gallon.I would almost go up to 4.5 really.But i would use a blow-off. I think the lower the temp the less likely it will go upwards of 10 degrees higher. I would assume about 5 degrees with low temps roughly on average.

Should I aim at getting my swamp cooler closer to 60-62* figuring the fermenter temp will get up to 5* higher?
I now have 5 buckets and plan on doing a batch week keeping the first 2 buckets in the cooler (large storage container) and allowing the last to reach room temp ~74* for the last week.

I do 6.25gallons in my 6.5 gallon bucket, always use a blow off and never have a problem.

So there is no reason to want/need the kreusen in my batch?
Can a problem arise from using a 1/4" blow off tube? I can see it possibly getting clogged somewhere along it's 4-5' journey to a jar. I'm thinking that maybe I ought to get a 3/8" ID tube instead and pull out the grommet from the lid, but the idea of trying to install a grommet later seems as though I'd be asking for problems.