Brother printer: check drum life before ordering new printer cartridge

I've ordered 3 printer cartridges since I bought my Brother 2140 a bit over two years ago [2]. Yes, we still print. It's a kid thing. The toner light is on again [5], so I'm toner shopping.

I don't mind buying them, the printer is reasonably economical and, ever since I set it up with an Airport Express print server rather than using my Snow Leopard iMac it's been trouble free. the cartridges aren't cheap though, a new cartridge is a good fraction of the price a new printer with its low capacity "starter" cartridge. [1] So I like to check the drum life first.

I thought I could do that through Printer Setup or CUPS (http://localhost:631/admin/?ADVANCEDSETTINGS=YES) interface, but I couldn't get it to work. This did:

How Do I Run a Self Test on a Brother HL-5240 Printer? | eHow.com

You can print a 'Printer Settings' page by pressing the 'Go' button on the front of the printer three times within 2 seconds. This page will provide information about the printer, such as its' media access control (MAC) address."

From the Printer Settings page I see I've printed 10,152 pages and I'm on my "fourth" (third really, first was tiny [4]) toner cartridge and the drum has 13% remaining life. That suggests a low cost printer drum is good for about four cartridge replacements. The current alternative is the Brother HL-2270DW or HL-2240D; they use the TN-450 printer cartridge; my current printer uses the TN360.

I bet I can get one more cartridge out of the drug, so this time I'll get the high yield cartridge [4]. After this cartridge is done, I'll get a new printer. [3]

[1] When you price a printer, always add the price of a standard cartridge to the printer price.
[2] Some scummy vendors are selling obsolete printers for about $300 on Amazon. I'm surprised the sleazy side of Amazon doesn't get more attention. 
[3] My LaserWriter 360 lasted about ten years, but I think it costs about $1000 @ 1992.
[4] I think I accidentally bought the standard rather than high yield cartridge with my last purchase. 
[5] If you tape over the clear plastic portal used by the toner level sensor, you can keep printing. There are many web pages that describe how to do this. The trick is to put the tape on the toner cartridge, not the carrier. When I put it on the carrier I tend to forget, and then run out of toner. This way I order a new toner, and wait until printing fails before I replace it.