Univ. of Chicago Press. Be more specific or if it was mentioned previously, to do so again is redundant. Accurate implies complete freedom from error or absolute exactness. An estimate is an approximation. Try to use a reliable estimate.
Effect is usually a noun that means the result, or the change itself .
The article a is correct before any h that is pronounced. Alternative implies a choice among two or more incompatible objects or situations. Use these alternatives instead. Marley's ghost appeared but seemed harmless. Do not use as to mean because or since.