Some appeals courts in Missouri, which expressly permits consideration of misconduct, have indicated that the fault factor comes into play only if the misconduct placed an undue burden on the other spouse or the marital partnership. In the most frequently cited case, Burtscher v. Burtscher , 563 S.W.2d 526 (Mo. Ct. App. 1978), the court said there should not be an inordinate focus upon a particular incident or even a series of incidents, particularly in a marriage of long duration. The conduct factor becomes important only when one spouse's misconduct changes the balance so that the other must assume a greater share of the partnership load. See, e.g., In re Marriage of Gustin , 861 S.W.2d 639 (Mo. Ct. App. 1993) (wife's act of chopping through door of marital residence after parties' separation did not place any burdens on the marital relationship); Divine v. Divine , 752 S.W.2d 76 (Mo. Ct. App. 1988) (husband's conduct - failing to communicate with wife, physically abusing her, harassing her over the telephone, and making false accusations against her-unquestionably placed undue stress on the marriage).
Moreover, even when authorized to consider marital fault, courts often refuse to give adultery much weight as a factor in equitable distribution. For example, in Barth v. Barth, supra , a Missouri appeals court declared that an inordinate focus on adultery is an inappropriate basis for a wholly disproportionate award. What is wholly disproportionate depends largely on the facts of each case, the court said, upholding a 55-45 split in the case before it. The court pointed out that in D'Aquila v. D'Aquila , 715 S.W.2d 318 (Mo. Ct. App. 1986), a 60-40 split was deemed disproportionate, but that in Schwarz v. Schwarz , 631 S.W.2d 694 (Mo. Ct. App. 1982), an 85-15 split was upheld. Adultery has been accorded substantial weight in Missouri cases involving repeated adultery, e.g., Cook v. Cook , 706 S.W.2d 606 (Mo. Ct. App. 1986) (where husband had been repeatedly unfaithful to the wife, appeals court upheld award to wife of 86% of the marital estate), or physical injury. Mastin v. Mastin , 709 S.W.2d 545 (Mo. Ct. App. 1986) (husband's misconduct burdened wife sufficiently to justify a property division heavily weighed in favor of wife, who contracted a venereal disease from him twice).
You may want to research if there is more current case law but I wouldn't get your hopes up that her affair will result in any sort of windfall for you. It looks like it will largely depend on which circuit you are in and which judge you get and no one could accurately give you an estimate on what the division would be other than 50/50.