FWIW most empty rooms with out domestic furnishings such as rugs, drapes, pictures, furniture, etc will have echo slap. Don't let that overly concern you.
Apart from room induced nodes in the bass created by room dimensions, most of your room problems can be controlled by two things; 1)deadening the first reflection points on floor, side walls, and ceilings (don't overlook the latter if you don't have WAF's), and 2) put acoustic materiels on the wall behind the listening position which diffuse/dispurse the mid/high frequencies I'd be concerned that if you have treated the first reflection points well that using deadening materiels might overdamp your room. You might also put similar materiels behind the speakers but I wouldn't do both and, unless I had panels or electrostats, I probably wouldn't do much to the wall behind the speaker if I did all the other stuff.
Bass nodes and nulls will be determined to a great extent by where you place your speakers and listening position. Trying to anticipate these issues in advance by determining exact room dimensions by a formula is an invitation to disaster, especially if you do it with out an expert on-site to help, and even then I've seen a few 'expert' disasters in special built and treated rooms.
IMHO, in a rectangular room there will always be some bass nulls and nodes and all you will accomplish by changing dimensions is to move them around frequency wise. If you have a gross node you can always use a parametric type equalizer of the digital or analog type or if the problem is in the upper bass you can probably get some good results from using bass traps etc.
Hope that helps a bit and gets you pointed in a good direction. I'm sure some of the experts will opine as well.
Apart from room induced nodes in the bass created by room dimensions, most of your room problems can be controlled by two things; 1)deadening the first reflection points on floor, side walls, and ceilings (don't overlook the latter if you don't have WAF's), and 2) put acoustic materiels on the wall behind the listening position which diffuse/dispurse the mid/high frequencies I'd be concerned that if you have treated the first reflection points well that using deadening materiels might overdamp your room. You might also put similar materiels behind the speakers but I wouldn't do both and, unless I had panels or electrostats, I probably wouldn't do much to the wall behind the speaker if I did all the other stuff.
Bass nodes and nulls will be determined to a great extent by where you place your speakers and listening position. Trying to anticipate these issues in advance by determining exact room dimensions by a formula is an invitation to disaster, especially if you do it with out an expert on-site to help, and even then I've seen a few 'expert' disasters in special built and treated rooms.
IMHO, in a rectangular room there will always be some bass nulls and nodes and all you will accomplish by changing dimensions is to move them around frequency wise. If you have a gross node you can always use a parametric type equalizer of the digital or analog type or if the problem is in the upper bass you can probably get some good results from using bass traps etc.
Hope that helps a bit and gets you pointed in a good direction. I'm sure some of the experts will opine as well.