Over ten years ago I did up this OB for the two armies:
Henry Tudor: three battles in line:
LEFT 2,000 Welsh, half archers and half light and medium spearmen under the command of Rhys ap Thomas (morale "D" all)
CENTER 1,500 "French" mercenaries, one-third light infantry crossbow, one-third light billmen and one-third medium and heavy dismounted men-at-arms, under the command of John de Vere the earl of Oxford (morale "D" all)
RIGHT 1,500 one-third "exiles" light, medium and heavy dismounted men-at-arms, one-third "freebooters" light, medium and heavy infantry, and one-third "Scots" mercenaries light infantry "spearmen", commanded by Sir Gilbert Talbot (morale "C" "exiles", "D" "freebooters", "C" "Scots")
RESERVE 250 heavy mounted men-at-arms, commanded by Henry Tudor. (morale "C")
Richard III: two battles in column on high ground:
VAN 3,000 one-sixth dismounted heavy men-at-arms, one-sixth medium crossbow, one-sixth light crossbow, one-sixth light longbow, one-third unarmored longbow, commanded by John Howard the duke of Norfolk (morale "C" all)
MAIN 5,000 three-tenths dismounted heavy men-at-arms, three-tenths medium "billmen", one-fifth light "billmen", one-fifth unarmored longbow, commanded by the king, who is with a mounted reserve of 500 heavy men-at-arms immediately behind the main battle. (morale "C" all)
Henry Percy the earl of Northumberland has 3,000 men one-sixth mounted heavy men-at-arms, the rest light, medium and heavy infantry spear/bill. (morale "D" all)
Thomas and William Stanley have 3,000 men one-sixth mounted heavy men-at-arms, one-sixth dismounted heavy men-at-arms, one-sixth medium "billmen", one-sixth unarmored longbow, one-third light "billmen". (morale "C" all)
The earl of Northumberland's command may be left off-table to the king's rear (saves having to use figures that way if you're short.) What he does is key to what follows. When king Richard commits the main battle, Northumberland will make a test to see if he stays put or rides off the field. You can make this a 50/50, heads he rides off the field, tails he stays put.
If by some method king Richard can avoid committing his main battle, i.e. defeat Henry Tudor with just his van battle, then Northumberland remains in place well to the rear, and the Stanleys also remain in place.
But if Northumberland quits the field, when Richard commits his main battle, the Stanleys will join the king's army and attack Henry Tudor.
But if Northumberland remains in place, after king Richard commits any part of his main battle or reserve, then the Stanleys roll 1d6: 1,2 = they join the king's army and attack Henry Tudor; 3,4 = Stanleys remain in place; 5,6 = Stanleys attack king Richard
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