Auctions are a dodgy business, as you can never tell where the price will go.

"Guiding" is cynical , a sprat to catch a mackerel. Don't you love the "guiding" bit? As if estate agents were caring welfare officers, utterly devoted to providing a guiding hand to the lame, the blind, the inebriated . . . ? And to those of unsound mind.

As agents often tell you, more in (feigned) sorrow than in anger: "It all depends on the day, it only takes two determined buyers in the room to drive the price up." Do not be misled by their furrowed brows at your concern. Not only are they good fiction writers, they are also good actors and will never be without a life supplying scripts and acting auditions to Ardmore. From their point of view - and of the seller - the more bidders, the merrier will be the agent's commission and louder the seller's laughter all the way to the bank. They are also quick to tell you, their primary loyalty is to the vendor.

To counteract this "culture of selling", to resist their helpful, manipulative "guidance" my own policy is never to go beyond the figure I have in my head as to what the property is worth to me. At the end of the day, I have to get my calculations right. If I'm going to be the purchaser, I'm the guy who is paying the bill - I know from experience the bill is always higher than first seen on paper.

I have also learned the final figure will invariably be higher if I like the property. Rather like my best female friend who is so enamoured of a dress - and how she might look in it - that squeezing her ample size 14 into the stated label size 12 is a mere trifle to be ignored. Mind over matter, she says. So with a desirable property. The more imbued you are of its inherent qualities, the more your mathematics runs awry.

That is where "unsound mind" intrudes - I have known reasonable people, in many respects savvy, sensible and not given to excess - throw their character out the window when faced with potential defeat by another bidder at auction.

So, I have learned to do basic sums, well before the auction, and not let emotion cloud the figures on the day.

For instance, allow another quarter on the "guiding" price - add a third if there is a lot of interest. Then, how much more will I have to spend in the way of refurbishment, in order to get (and hold) quality tenants? How much more for internal outfitting? Add those sums to the selling price, add another 10 per cent to round up stamp duty and legal fees. Now, we're heading towards a realistic figure - usually well above what the agent is "guiding".

As to the actual finance, quick calculation on the back of an envelope tells me I will need to borrow, say 80 per cent of the purchase price at, say, 5 per cent over 10 years. What will be my repayments?

Never mind what the bank says - the reality a few years hence is always more than banker initially tells. More sprats for mackerel. A 0.25 per cent increase in interest rates can have you ransacking the holiday money to keep up your repayments. So, add another percentage point to loan repayments. Crucial question: will rents meet those repayments?

The sums say, yes. Being mere numbers (inanimate objects), sums have no way of accounting for human frailty and variety. Sums do not factor void periods, such as two tenants, whom you assumed hardly knew each other, striking up a passionate relationship and deciding to live as one - somewhere else - on the day rent was due. Or the quiet guy from the inner city, whose motorbike you noticed in the car-park, disappearing without trace.

As you tell the two plainclothes men who will spend a few days going over his (your) flat, taking up a few floorboards, probing walls and eyeing you suspiciously.

All such experience - and sums - tell you keep away from auctions and buy at private treaty sales where you know the likely outer limits of price - and can calculate your own cash outlay.

Of course, if you have the property bug, you will ignore this advice. You will be among the many hundreds at auction today. I will probably be in the seat behind you, doing doodles on the back of an envelope, working out my final price, beyond which I will not be "guided".