Is there any length to width criterion for the centre, for example in a 250 x 50 feet plot?
There is no criterion for finding the centre; the centre is always the centre. The proportion matters for the plot itself: up to 1:1.5 is good (30 x 45 feet is ideal), up to 1:2 (30 x 60) is neither good nor bad, and longer than that is not good, with more loss the longer it gets. In such long plots many devtas fall outside or get cut, and they need treatment.
Related questions
How do we count the length to width ratio of an L-shaped plot, for example 20 x 20 feet at the back and 10 x 30 feet in front?
An L-shaped plot is inauspicious in the first place. There is no special formula for its ratio; estimate it from its arms. The cut portion is a full loss, and how harmful it is depends on which corner is cut, which is best judged with the teacher case by case.
If the built-up area is on one side, such as South and West, with the other sides open, how are the pishach bhoomi and devtas found?
Use the angular method taught in class. Whatever the tilt and whatever the shape, octagonal, pentagonal or regular, find the centre, divide the plan into pishach, manushya-dev and brahma bhoomi and mark the pads by degrees as demonstrated, then practise it from the recording.
Is a highly metallic, mineral-rich soil good or bad for a land?
It is bad. Where metallic minerals, and especially black ones such as coal, are plentiful, negative forces settle: mineral-rich Bihar and Jharkhand and diamond-bearing Panna district are not prosperous, because few people can bear such strong energy, just as a 400 carat neelam or 20 paracetamol harms instead of helping. Minerals at the surface act on the house directly, while metals bound with silicates or lying deep underground, as at the Kolar Gold Fields, are not as harmful.
About 11 or 12 of the 45 devtas are asuras. Whose energy should be increased and whose kept low to stay balanced?
This was to be explained in detail later, as there is a big confusion about it. Asura devtas are also needed: the day there are no asuras in your house, your work will not run, so the aim is balance, not removing them.
After placing sweets and later chanting the mantra 27 times at a devta place, I felt strong heat in my body. Why?
It shows a defect or imbalance at that pad. When an offering reaches a defective place the defect tries to come out, as when granite tiles at one devta pad cracked during a group havan, and the body reacts. The uneasiness is a signal: the pad must be purified and activated with the method being taught (havan samagri, bhog samagri, mantra or the samagri to hang), so wait for the full method before trying again.
An 80 x 50 shed has its floor sloping North but its roof sloping West. Is that fine?
No, the westward roof slope is wrong. It disturbs the energy current, so some trouble will persist there, and the roof slope has to be corrected.