How do you estimate apparent attenuation

[PaulStat]

In brewing software it asks you to enter your apparent attenuation, from this it goes off and calculates the estimated FG and ABV. Problem is how do you know what the attenuation is going to be, I'm keeping the default 75%, but with the leffe blonde clone I'm planning on doing it calls for 900gms of sugar. I think it was Steve who said your apparent attenuation for that amount of sugar will be higher which means the figure calculated by Beer tools pro for the ABV is wrong.

So how do I know what % I should put?

[PaulStat]

Dunno if anyones interested but I did find out how to calculate the apparent attenuation, it's not the estimate however.

Specific Gravity & Plato Scale
A solution's specific gravity (SG) is its density (g/ml) relative to water, and is easily measured with a hydrometer or other suitable instrument. Wort (unfermented beer) has a specific gravity greater than water due to the presence of sugars. Beer has a specific gravity less than wort because some of the sugars have been fermented into alcohol. Professional brewers often use the Plato (°P) scale, instead of specific gravity, as a metric for the sugar levels in wort and beer. The Plato of a solution is equivalent to its percent by weight of sucrose and has dimension of (g equiv. sucrose)/(100 g solution). Thus, a 1% sucrose solution is a 1 °P solution. For the same weight of other sugars, the Plato of a solution is slightly different. The relationship between Plato and specific gravity is nonlinear.

Jan DeClerck [A Textbook of Brewing, 1957, reprinted by the Siebel Institute External in 1994] gives a least squares fit for conversion from specific gravity to Plato at 20 °C. DeClerk's equation is used for all subsequent calculations below since it deviates from the values given by the ASBC ["Table 1" in: American Society of Brewing Chemists, 1992, Methods of Analysis of the ASBC. American Society of Brewing Chemists.] by less than 0.04% °P from SG 1.010 to 1.083:
(1) °P = (-463.37) + (668.72 × SG) - (205.35 × SG^2)

From the plato scale you can then go on and calculate the AA based upon the Pinitial and Pfinal

Attenuation
Attenuation is a measure of the degree to which sugar in wort has been fermented into alcohol in beer. Ceteris paribus, a sweet beer has more residual sugar and lower attenuation. Hydrometer measurements of the specific gravity before fermentation and after fermentation are used to determine attenuation. However, the residual sugars are not in a solution of pure water; rather they are in solution with water and ethanol, which has a density of 0.79 g/ml. Thus, many brewers give a number which must be called the "Apparent Attenuation" (AA):
(3a) AA = 1 - [°Pf / °Pi]

[Jim]

I would say the chances of correctly estimating your attenuation in a grain brew are minimal - there are too many variable; yeast strain, pitching rate, mash temperature, proportion of malt to sugar etc etc.

You might as well just leave it set to 75% - that'll be near enough.