so optimal bpt is:

sqrt(acres to be built/4*govt bonus)

and this caters for someone starting with a bpt of 5.

what if you're rebuilding say from a series of ABs, how do you cater for a different starting bpt?

It's the same formula.

sqrt(acres to build / 4 * govt bonus)

where "acres to build" is exactly that, usually the number of acres you have to rebuild, plus any further land beyond that you intend to grab. I'll even give you an example, taken from a guide I wrote a long time ago in 2002 (when Theo Techer -> Theo MBR was still viable).

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Suppose I have 20k acres and 117 BPT as a Theo techer (316 CS). While I am stocking, a suicider ABs me and kills 70% of all my buildings. This leaves me with 14000 acres to rebuild and 94 CS remaining, giving a BPT of 39. I wish to find out what Ideal BPT to rebuild to, to rebuild all 14k acres in minimal turns. Ideal BPT = Sqr(14000 * 1.4 / 4) = 70 BPT. So building my BPT up from 39 to 70 (which is 180 CS, so build 86 CSes), then rebuilding the remaining acres will give minimal turns. If you do not believe this is true, substitute in the numbers and you will find that building to 71 BPT or 69 BPT will use up more turns. If I wished to still convert to MBR, the AcresToBuild would have included in the extra 19700 or so acres. Usually, in such a scenerio, it is better off to actually convert to MBR right away unless the end of reset is still very far off.
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interesting. useful formula. thanks =)

why is TTR -> TMBR not viable these days? I've been told that coming back, but haven't found out why yet.

1 reason is due need to rebuild and it takes forever if has alot land (50k+) and costs big fortune (even building 20k acres costs over 2b with normal building cost), I think Xinhuan said somewhere that if has under 20k acres then mbr is viable.

The formula that is used to calculate how much it costs to build 1 acre was changed from

3*land+1500

which is a linear formula, to

3*(max(1,land-1500)^1.05)+1500

on 2013-04-06 ( http://www.earthempires.com/...hanges-23941?t=1376061607 ). This is a polynomial formula (not exponential, but they both mean the same thing to most people). This was subsequently reduced to

3*(max(1,land-750)^1.03)+1500

later on 2013-08-06 ( http://www.earthempires.com/...resets-25801?t=1382991109 ), which is cheaper, but still significantly more expensive than the original costs. This meant that converting to MBRs incur a much higher rebuilding cost (and higher tear-down cost), to the effect that it is too costly to perform compared to any benefits gained as a TMBR.

This set of changes were aimed to combat landtrading on the Alliance server, in response to this reset ( http://www.earthempires.com/alliance/20/ranks ) where the top 33 players were ALL landtraders when LaF decided to show the world how Landtrading should be done after years of arguing that "Landtrading is the bane of this game because it is overpowered and that it shouldn't be allowed". Failing to convince anyone, LaF spent 1 reset proving just how broken it is when every other clan out there suddenly has 2x-3x as much landtrading partners to trade with.

lmao so that's the reason for that, and why top acreage has been greatly reduced. i guess it's a good change. it's cute of LaF to do that. =P