Back-test comm & slippage: always add in the spread?

I'm running my algorithmic trading system through some back-tests and I'm trying to decide what costs to add to the profit or loss of each trade.

Obviously the commission comes first - with InteractiveBroker it's £3.50 per round-turn.

Secondly there's slippage which depends on the market, the way the market is moving, the type of order placed, the way the wind is blowing etc. I originally decided on $25 per round turn but I'm not sure yet. Haven't traded on IB before.

Thirdly, my question: will I always pay the spread? e.g. going long, I'll buy at the Ask and sell at the Bid?

So if I trade FTSE100 futures, I'll be paying 1 tick or £10 per round turn, right?

So when I come to work out my system's profit or loss, ignoring slippage for a moment, 100 trades are going to cost me £350 commission plus £3000 for the spread?

Or is that not so simple?

Wouldn't it be more realistic if the slippage was set as a percent of each share on market orders so that your able to see the effect on larger orders and smaller orders equaly? Or if limit orders it seems that just requiring the price to move one tick further than your limit to ensure you actually get a fill.

Wealthlab does it, and it makes sense

Yes, you're right, but I meant buying / selling 1 futures contract - so if I traded more than 1 lot, the effect would be just multiplied up, so I meant the same thing.

I just realised my arithmetic on that first post was wildly wrong!

At £3.50 per contract (FTSE futures with IB), the spread at 1 tick or £10 per round-turn, 100 trades will cost me

Commission:....£350
Spread:............£1000
Slippage:..........£???